It's not just completing the game, 100%ing means you've finished everything. Got every collectable, finished every sidequest, and pretty much finished everything the game has to offer. To me, that should mean you achieved everything the game has to offer.
Sure, there can be a few achievements for doing something harder, but it shouldn't reach a point where it feels like an unfair challenge run that needs hundreds of attempts and thousands of hours to beat. There isn't an achievement for beating dark souls without taking a hit, but people do it for the love of the game.
For people telling me to ignore the ultra-hard stuff, it's really demotivating to want to get all the achievements but be blocked by blatantly unfun challenges like "beat the game on ultra hard difficulty on ironman mode where if you die, you lose 20 hours of progress and have to start over".