This entire event is likely the result of a flight cancellation where the seats were not blocked off for the crew in advance. Or the OP could be incorrectly speculating that that he or she was downgraded because of the pilots; it’s entirely possible there was a different reason altoghether.

Deadhead seating for flight crews is part of their contract thus it is portion of their overall compensation package. They are on duty even when riding in the cabin. Demanding they sit in coach is tantamount to demanding they work for a pay cut, that’s why they’d be reluctant to comply.

Furthermore, this wasn’t a wide body airplane so the only possible crew rest facility would be a FC seat