I do think think people underestimate the impact of titles & labelling of the games.
A lot of people simply aren't interested in messing with a franchise so deep into it's sequels. Especially a narrative driven one where the average Joe assumes they're connected. A lot of us grew up with FF in its prime but gen Z grew up with XIII saga, a whole bunch of bad spin offs and XV.
Then as far as the remakes go, naming them Remake & then Rebirth does zero favours to the marketing. Just call it part 2, part 3 lol. I actually think VII Rebirth success is less of a issue, I think it was probably one of the cheaper entries in recent memory (less than 4 years development, just above 3 if we count from Intergrade DLC). It's more what it means about appetite for the franchise as a whole.
We don't have numbers but it looks like it performed maybe slightly worse then XVI (+6% in the UK, - 30% in Japan, probably up a tad in US similar to the UK)
As someone said a soft reboot would be good. Maybe a naming convention by theme instead of number. Zero spin offs to avoid watering down the brand. Slightly smaller scope entries (30hour story, essentially VII Remake length more story) with the same scale of visual/audio production as we're used to. Unified production platform so developers are not starting from scratch. VII, XVI & Forspoken are built in completely different engines. A game like VII for example could of benefitted from Forspoken terrain navigation system. Back to exploring turn based/tactical combat a way to diversify.
PC/Xbox/Playstation day one. Switch 2 ports a year or so later