Definitely a fair criticism, I'm not sure either. I was thinking that they have a sort of movement option already so it MAY (huge may lol) be possible to take that core concept and code, modify it slightly, add it as a new option in the menu and call it a day. However I feel like going deeper could add a lot of immersion and fun experience shift.
It would also depend on how they approach it, perhaps a clawed animal goes up hills faster when on the simulated movement mode. Cars go down hill faster. Or a mount has simulated mode only abilities that boost it's speed but it goes slower without them (faster in whole if you do well), so you can play a little mini game (nothing complex but something different to experience). In that scenario I think a lot more people would keep it on, to play with the new toys (and more immersive visual).
I don't want massive resources redirected to the system though (unless they can justify it), just if its something one person at the company finds interesting to work on as a passion project or if they can make it happen quickly with systems they've got going on already.
It might be justify able if they can see it being useful for other content in the game. Like if they decided to delay flying in a zone to even later but wanted players to experience making a jump with a rocket boost off their wheeled mount (one given to the player just encase they don't own Falcon, SDS, or Regalia). Which I think could be great as it serves to make movement more enjoyable before flying and also give each zone a greater time to shine and unfold before being stripped to reveal all.
I could see a Eureka like zone using something like this to great effect. That'd be awesome. We keep flying for most zones but a few zones gain a new environmental progression. I could also see if they delay flying in zones that we might get good use out of that. Potentially a lot of chocobo content in that concept too!! (/dies for chocobo content).
One of the things I don't know if other people miss but Chocobos in a few other FF games were able to jump to some hard to reach locations. I get that feeling when I think about this system and your idea about relic and stuff.
Sort of miss those cute chocobo hops lol.