armies must be saying about their discarding of their Geopard SPAAG's?
MAD
It mostly came down to employment & range. Many 35mm were assigned to point defence, say a bridge or an airfield. Since their range was around 2.5-3km, they were often co-located with the target. As helicopters started to be equipped with missiles capable of a 8km stand off, the guns became a last resort option within the mix of AD ranges, and became focused on anti-radiation missile defence using AHEAD type ammunition. VSHORAD role was replaced by missiles in the 90's, and most SPAAG were equipped with a bolt-on VSHORAD missile.
Then VSHORAD kind of became taken over by SHORAD systems with a wider engagement band.
Nowadays, with the ability for RPV to operate in contested airspace, the idea of a more cost-effective, mobile, kinetic killer is back on the table.