I wonder if keeping their GPUs mining until the last minute ended up making those miners more money than selling them when the price of GPUs was still high would have.
Not at all. Personally I got out and sold my GPUs in March and April. Anyone still mining these past months was either lazy or betting the merge would be delayed.
A delayed merge sounded like a good bet on paper. Eth has been promising PoS soon for years. It is understandable why some miners took the bet this time.
The problem with gambling in an iterated context: you will always lose eventually.
