I work for Allegiant, and every time I deadhead or jumpseat on AA or DL, I can't believe how much more cramped their Airbi feel compared to those of the ULCCs. They have a lot of wide seats crammed into those planes, but way less of them than us, yet the plane still feels cramped and the aisles are narrow. I really don't think this is going to affect the ULCCs as much as the legacy carriers. If anything for economies of scale. They have a lot more planes to retrofit if a change is made than us.
But most likely, in true FAA fashion, they will pick a number slightly below whatever the smallest number currently is, so no one had to modify anything, then announce to congress and the public how much they improved safety and comfort.