The fact is that this change will not 'assure that more music remains on the radio.' The way to do that is make better music.
"Better" music in whose opinion? Do you think if that hypothetically happened it would create additional music formats leading to the kind of diversity being discussed here? Or even an increase in the size of playlists in the current formats?
I think you know the answer to this. There's a ton of "better" music already out there that radio doesn't play, and even if it were doubled it still wouldn't diversity commercial radio.
The point here is about eliminating simulcasts where two FM signals in the same place should be carrying separate programming. Now, I don't even know if, or where, any such simulcasts are currently happening. Why would a station owner even choose to do such a thing? Once scenario I can think of would be to cut the cost of programming two separate formats but still retain control of both licenses rather than allow the unneeded one to go to another broadcaster. That would certainly not be in the public interest and this simulcast rule should prevent it from happening.