Well, I'm the same way. Once I got 90+ lives I never dropped below 90. The closest was in Superstar Road. I think even NSMB Wii had me drop to 85 one time or something.
But just because I'm adept at the game doesn't mean I can't detect the challenge. I'm good enough that I can beat the game without too much trouble, but I still notice when it's pushing more complex challenges on me with the level design. It's not a giant leap in difficulty, but the change is certainly apparent.
Basically, it's not that I had a hard time. I never had to put more than a few tries into a stage, with the exception of a couple of Superstar Road's stages when I got cocky. That said, I recognize that I'm generally good at these sorts of games, and I'm well-trained after beating NSMB Wii, Galaxy 2, and 3D Land 100%, but that doesn't leave me blind to the idea that the game will be noticeably harder for a lot of other people.