I'm the opposite of OP. I feel that changing systems in different games in a series is necessary to keep things fresh. They don't change systems to "innovate for innovation's sake" and nor is it limiting the gameplay. And I feel the title and OP are not aligned properly, since the OP never even really argues or elaborates on that.

Now, that's not to say I'm against using the same systems in a series. Legend of Heroes essentially does it and I dig that series and combat. It's basically 2 separate approaches to appease 2 groups of ppl - those who embrace change/enjoy variety and those who do not.

It sort of reminds me of fighting games and how new entries in a most series change things up. And ppl would complain about it, well after a new entry's release. And the general retort would be "go play (that older) game then."

Of course, competitive pvp game series aren't the same as SP RPG series, when it comes to what is required for the optimal player experience, i.e. FG's you sorta need ppl to play, preferable a variety, and accessible platforms to play. So "just go and play KoFXI if you don't like KoF15" is sort of suboptimal advice whereas "just go play FFX if you don't like FFXIII" can kinda be decent advice? But if you're playing an RPG and don't like the systems, but want to keep playing for the story, then just watch the "as a movie" video on YouTube.

Jeez...is this why 90% of the SMT fans I know basically only play SMT? And over and over?

I don't know...the more I think about this topic, the more I feel it's kind of asinine. Especially for a series like FF where every main game is a separate story/world. It's not like you have any sort of sentimental attachment to anything in a new entry.