A gearbox does not a reliable tank make. Besides, just because something is the technological basis for a later development doesn't make it inherently reliable. The V-2 rocket was the ancestor to the Saturn V, one of the most reliable rocket designs in the history of spaceflight, and I don't need to tell you how reliable the V-2 was...
In Italy, the 21st Tank Brigade (12th RTR, 48 RTR, 145 RAC) operated a mix of Churchills and Shermans in operations together. From late 1944 to early 1945 the Brigade was involved in assaulting the Gothic Line. During those operations, the losses were 52 Churchill, 29 Shermans, 4 Stuarts of which 15 Churchill and 21 Shermans were recovered in salvageable condition and repaired. The disparity between the Churchill and Sherman write off figures can be explained by the fact that the former were used primarily as assault tanks and the latter as support tanks.
In maintenance the Brigade workshops used:
- 23 Churchill engines
- 4 Sherman engines
- 1 Stuart engine
- 6064 Churchill track shoes
- 240 Sherman track shoes
- 87 Churchill bogie units
- 24 Sherman bogie units
- 4 Churchill gearboxes
- 7 Churchill final drives
- 2 Churchill idler wheels
- 28 Churchill drive sprockets
While the Churchill gearbox seems to be the least troublesome component on the Churchill, it still falls well short of the Sherman for reliability. During that time, they didn't need a single replacement Sherman transmission.
I hope this example puts the matter to rest. If we can't evaluate the Churchill as it existed in 1943, 1944, 1945, or 1951, I have to ask at what point we can evaluate its reliability? The Shermans used in Korea were likewise well worn, some even having been used as monuments before being reactivated and sent to Korea, but they served reliably throughout the conflict.
As for the Churchill's role as an assault tank, it was well suited for the role, certainly, but I'd argue that it was no better suited than the Sherman Jumbo. You see, while envisaged as an assault tank, the Sherman Jumbo ended up not being used in that role exclusively. It was the versatility of the Sherman that we've been discussing that permitted the use of the Sherman Jumbo in ways that would not have been wise or economical for the Churchill VII to perform.
You see, you're looking for a Churchill-sized hole in the US Army for the tank to fill and—because of the way the US Army was organized at the time—no such hole of any size existed at the time. US Shermans operated in two primary formations during WWII, Armored Divisions and independent tank battalions. The Churchill didn't have the speed to operate in US Armored Divisions given their role of exploitation. The Jumbo, however, could keep up—and did. In fact, the Third Army assigned more than half of its allotted Jumbos to Armored Divisions.
Now, I imagine your proposal is to equip some US independent tank battalions with the Churchill, operating in the role of infantry support tank as it did in the British Army. There are two main issues with this. First of all, US independent tank battalions were not permanently assigned to infantry divisions. Some were shuffled around to different sectors of the front, supporting different infantry divisions, airborne divisions, cavalry squadrons, etc. Even if we ignore the logistical problem of trying to follow around a half-dozen Churchill battalions in US Armies, you still have the problem of different infantry division commanders having to adapt to operating with an entirely unfamiliar tank type. You mentioned the US Army eliminating the artificial division between tank destroyers and tanks after WWII? They had enough trouble with US commanders when they'd swap an M10 Wolverine unit for an M4 Sherman unit, and those were vehicles with VERY similar capabilities. Now try to swap an M10 unit for a Churchill unit. It would be a complete culture shock. You'd have the division commander asking the Churchill commanders to perform missions they were entirely unsuited for, or vice versa. I'd hate to show up to support a division with M18 Hellcats when the infantry commander was used to Churchills. It's hard to think of "tanks" that were more dissimilar in WWII.
Furthermore, the Jumbo never equipped any full battalion. It always operated with normal Shermans. The most Jumbos received by any single battalion is 15 tanks, about on quarter of its medium tank strength. This was no problem with the Jumbo since they could use the same tracks, bogies, engines, transmissions, etc. as any standard M4A3. You could not do any of these things with a Churchill.
If, on the other hand, you try to equip an entire US battalion with them, you run out of Churchills pretty quickly. You need 59 Shermans for a US tank battalion, and that's not counting attrition, combat losses, etc. They only made 1,600 A22Fs total. You use up about 4% of the total production run for every battalion you wish to outfit...no losses. Jumbos served in 13 different independent US Army tank battalions in the ETO, primarily used to lead assaults, lead tank columns, etc. If you tried to replace all the Shermans in those battalions alone with heavy Churchills, you use up half the A22F production run. The US deployed 35 independent medium tank battalions in support of the 42 infantry divisions that served in France and Germany. Replace all the Shermans in those battalions and you use up every A22F built, again, no replacements, none left for the Commonwealth.
If you try to operate them in penny packet fashion, as the US did with the Jumbo, then you run into all the problems with the logistics and maintenance at the lower level. A typical number of Jumbos in a battalion was 5. Give a US tank battalion 5 Churchills and see how much fun they have operating them alongside 50 Shermans and 17 Stuarts.
So, I reiterate, the Churchill VII could not have performed the role of the Sherman Jumbo in WWII. It wasn't versatile enough to serve in the variety of roles performed by the Jumbo. Mechanically, it prevented interoperability with the US tank battalions in small numbers like the Sherman Jumbo did. On the other hand, it wasn't numerous enough to completely replace the Sherman in anything but a very small number of US independent tank battalions. Even then, it would complicate the US ability to swap battalions between divisions as necessary risking even greater danger to the units operating the Churchill, the infantry they'd be supporting, and even other battalions that may be called upon to replace them.
I think the only reasonable scenario for the replacement of standard Sherman gun tanks in US Army service with Churchill VIIs on any scale is in Italy. If you replaced the Shermans that served with the independent tank battalions in Italy, you'd probably at least get an even break for the trouble. Italy was already the red-headed stepchild of the US Army logistical branch, never getting the good equipment like 76mm-armed Shermans, M36 tank destroyers, or Jumbos in any numbers. So, the armor would be a pure upgrade and the armament wouldn't be any worse. Further still, the Sherman couldn't make good use of its greater speed in Italy and the "mountain goat" nature of the Churchill would serve it well.
Finally, you're not talking about 35 battalions in that scenario. You're really just looking at 751st, 752nd, 755th, 757th, 758th, and 760th battalions. That's still over 350 tanks, 100 more tanks than the total number of Jumbos built and over a fifth of all A22Fs produced, but a lot fewer tanks that would be required in France and Germany. Alternatively, you could just keep all the Shermans and just replace the light tank companies in the independent tank battalions in Italy with Churchills. That allows you to basically cut out an unnecessary tank type (the M5 Stuart) that nobody's really going to miss and still gain the advantages of the Churchill. It also means that you only need six companies of Churchills, or about 100 tanks. Even with the necessary replacements, it's still a relatively reasonable number. Given how closely US forces in Italy operated with British forces and how rarely they got new equipment anyway, this relatively uncommon tank wouldn't be too much of a headache in Italy.
That's about the only plausible scenario I see for gun-armed Churchills in US Army service in any numbers in WWII.
Cheers,
Logan