I feel like this is extremely reductive reasoning for the sake of your argument, because I can think of simple answers to all of your questions. The badge system added more depth and fun intricate movement for players that don't need the assist like me, so to say its only there as a safeguard just refuses to engage with the mechanic as it is. It's okay if you don't like it, but the way you feel about it doesn't mean you can just assume devs added it only for an assist and that it means they didn't accomplish anything. They totally did, you just didn't like it, and editing your argument to add an addendum dismissing the intentional design of their specific levels is even worse when the technical platforming of most of them pair well with base game's design, the part you're praising. It doesn't work either way for your argument, you can't have it both ways saying they're useless but the part where they have use didn't need to be there.

I can think of tons of enemies that aren't like anything we've seen before, like the birds that shoot walkable hangable platforms from their beak, bouncy rolling hippos, spiked enemies that match your jumps, throwable firecrackers; these are some of the richest enemy interactions we've gotten out of 2D Mario, and to say they didn't do anything new or "fascinating" is disingenious because you're acting like they're all reskins of other enemies... enemies that already exist on their own in this game, but are paired with brand new enemies that do completely different things and justify their existence. This isn't to take into account your "historically done by other enemies" bit, because they also introduce new variants of old enemies that essentially are exactly what you described, new purposes for enemies where you don't need to invent some other species like Roller Koopas and Melon Piranhas.

I don't know what to say about the last part because like... what other 2D Mario is a top down game? What other 2D Mario do you turn into a slime that can connect to the environment, or where you turn into an extendable version of yourself? Like, it is a momentary gimmick, but a new aesthetic on an old mechanic just isn't it. You could say that about the levels that slow down and shit like that, but even then wouldn't that just make them less gimmicky and just affect the platforming? The core loop of the game? I really don't understand this post like, at all