As long as the spirit stays there, I don't think it matters too much. Unfortunately in a lot of cases, only some parts of the spirit are kept while others are discarded.

There's a difference between changing and furthering, and that's what developers don't seem to understand a lot of the time. It should never change so much as to be completely different, but should expand and become better. At its core it should remain largely the same, but a better iteration every time.

Just enough to keep it fresh while maintaining some of the core mechanics that made the formula so good.

For me the best example in recent history is Call of Duty. MW was incredible. Then Treyarch got a turn, they kept the core experience the same, sharing many perks and gameplay elements. But they also changed it up just enough that it was fresh. Then MW2 came, and even though it kept some of the same fundamentals, they went way overboard with some of the stuff, and birthed a whole new meta for the franchises MP.

I disagree. 

Tbh, it shouldn't. If the franchise is popular, it became popular due to what it did, refining those mechanics/graphics/etc... is absolutely part of the evolution of a franchise. But, completely flipping the script/mechanics is stupid.

If it's that big of a change, why wouldn't you just make a new franchise?

^ this is my belief.

you want to keep it fresh but maintain gameplay and story quality. maybe add a new gameplay element every few years, and always have consistent graphical design. if you hit Federation Force, youve gone too far

I don't mind subtle or drastic changes. Some are good and fun like Jak II and some are awful like Bomberman Act Zero.

XV will make FF great again.

FF never stopped being great.