Overhead lockers on our flights were pretty full.
First, it looks like plenty of people buy the priority boarding option - subjectively about 1/4.
Aside from that, everyone is still permitted a "small bag" which, practically speaking, was not all that small - many were decent size backpacks/bags. As long as it was not obviously too large and had no wheels, it could be brought on board. Since those also go into the overhead bins, there was not a lot of space on a full flight.