The game looks good. The whole package sounds like a "lighter" take on the franchise, which is cool by me. If you don't try to look for some aspects you cherished so much in the previous installments that are now missing you might enjoy the game more - that's probably an odd approach to adopt when playing a new installment in a long-lasting series, but it's also probably the most adequate one for this game. To clarify: TTYD is one of my favorite GameCube games, that game is BLAZING AWESOME and I am pretty negatively surprised about the lack of some set-in-stone staple elements such a party system, but maybe this sort of change will provide a welcomed different take on this franchise. Controlling Paper Mario traversing solo through that adorable handcraft-looking universe still sounds like an enjoyable thing to do. To give you another example, the lack of leveling up had me a bit worried until I realized that doesn't bother me in other Mario games, and those are still fun - I don't think the franchise feels significantly less unique because of this.
I agree: why can't they just let it be? Maybe they felt there wasn't much to add to the formula, and that any sequels would either be stagnant or have gimmicky additions to them? In any caseI don't see why you'd be worried about this game. It won't be anywhere near as good as TTYD, but it won't be anywhere near "not-good" either. To be worried about this stuff is kind of understandable, but also kinda silly. Game looks adorable. So cute. I want it.
It's been pretty well-received by the press so far, as well.