On the other side, there are PLENTY of big name pros that use a fat rim... The Bobo pieces all have thick rims, Jim Self, and then there is that gigantic Deck mouthpiece.
Not having a scientific survey of "big name pros" and their MPs in hand, I rely on the fact that, of the available models of MPs out there, the vast majority are not of the 24AW type. I would agree that the Yamaha Jim Self might qualify as a bigger 24AW. I don't agree that the Yamaha Bobo Symphonic qualifies. While there is quite a bit of metal there, it starts high near the inside and rolls away from the face so that maybe only the inner half (if even that much) is in your face. When you look at something like the Yamaha 66D4, not only is the rim wide, but it has a substantial relatively flat area so that most of it presses the face (and it has the student size 66 inner diameter, hence my recommendation for it to go on the list for student consideration).
So, now we throw into the mix the rim contour: Whether the rim is big because of a lot of rounding, or it is big to provide a relatively wide, flat support area for the face. Rim contour opens up another area of discussion.
some may feel that the thick rim is in the way, I used to.
My guess is that this is a common progression. And for the reason I stated.
For what it's worth, I never got along well with what seems to be the standards of Bach 18 / Yamaha 67. But I do just fine with the Kelly 18. I'd use it more, but I tend to get too punchy with it and stick out.
I think mouthpiece choice is a very important consideration for a student; and as the student progresses, the choice should be constanly reevaluated. I think the wrong choice will lead to the student compensating for the inability to use the MP correctly by doing something wrong and getting that ingrained into the subconscious -- like learning the wrong golf swing because you have the wrong clubs.