Lockers do sell out. Large sell out before small. Main Street sells out before other locker locations.
There is a higher chance of lockers selling out when there is a release of new merchandise that is large (ex. popcorn buckets, collectible dolls, etc.). In recent memory, Main Street lockers sold out on
D23 Day, the first weekend of the 2024 Holiday season/opening of TBA, the first weekend of the 70th celebration (in May) and on
Disneyland's 70th (July 17, 2025).
@Hutch1317 I wouldn't worry about the Main Street lockers selling out on a regular Saturday. Depending on what Disney decides to release, Main Street might sell out the first day of the holiday season (ie. Nov 14).
I think they changed the timing but the lockers outside of DCA used to close earlier than the other esplanade lockers. There was a sign just inside the locker bay with the hours. Definitely something to check out if you plan on using those lockers and staying late.