I do meme builds in PoE, ironmans in Fire Emblem, and nuzlockes in Pokemon. I can adapt my own difficulty, thank you very much, I don't need the game doing it for me.

If you're struggling on a boss, and it just keels over from a heart attack instead of fighting you, you don't get any sense of accomplishment or success from the encounter. If I wanted a cakewalk, I'd adjust the difficulty slider, or do something different on my end. If I wanted my ass handed to me, same thing in reverse.

Difficulty SETTINGS are... okay. Neutral. You can make a game as intended, or make several very similar games that have different balances, sure. But difficulty changing on the back end for no reason other than "adaptive"? It ruins fun runs, and ruins purposeful challenge modes.

FFX's no-sphere-grid run would be boring if the enemies all scaled down because you never bothered to level up even once. You're making a choice. Pick a slow car in Chocobo Racing 2? The fast ones SHOULD get quite a lead on you if you make poor driving choices.