I'm not a 'F-35s are lame' fool but I think keeping a fleet of new F/A-18s as a backup seeing as we already have the logistics array for them would have been sensible. Older aircraft do have a higher availability rate and keeping a high number of robust, older airframes available for non-peer warfare means you can shepherd your newer airframes. Also, to be blunt, we have too low a population base and economy base to maintain first line equipment in any meaningful numbers.