orbitmic

In the unlikely case that this is not obvious, large bags and suitcases get lockers priority over everything else, simply because there is nowhere else they can be put in the cabin. You (literally) can put a small bag under the seat in front (unless you are sat in row 1 or an exit row), you (literally) can keep your coat on your lap, but there is (still literally) no place in the cabin where you can put a cabin suitcase if there is no locker space left, meaning that it would then have to be checked in the hold.

That said, in literally thousands and thousands of flights I have taken over the years, I don't think that there has been a single case on a regular commercial aircraft where it would not have been possible to fit a coat "somewhere" in the lockers after suitcases have been put in, but on a busy flight, that will simply need to be on top of large bags/suitcases or in between them where it is more likely to be crushed.