Hhhmmm. I see a potential problem with this new experimental approach.

To pack all the carryon bags the most efficient way possible, smaller bags should go under the seat and the larger roller bags overhead. If people without roller bags board first, you will find that many will put their smaller bags overhead leaving the space under the seat empty. When the roller bag people board later, there will be less space overhead for bags and more bags to be gate checked.

Of course, this also makes it more important to have elite status.