Perhaps other passengers on this flight were in the same situation. They were maybe sat with their friends and family and didn't want to be split up.
Ryanair is a crummy airline. It's so crummy that after just one return flight with the airline (Gatwick to Dublin), I promised myself I would never fly with them again.
The cities they fly to are useless to just about anybody, their fares are not cheap, their policies in general are ludicrous and the way they charge optional supplements to receive treatment and services similar to the likes of BA is unacceptable.
Whilst I can appreciate the situation you were in, I don't think it's fair to take your anger out on your fellow passengers. At the end of the day, Ryanair is at fault here. The passengers already on the aircraft had no doubt turned up in good-time for check-in (in order to get a lower boarding card number) or paid extra to climb aboard before anybody else. Personally, like the other passengers on the aircraft, I would have been very reluctant to give up my seat, too.
For a substantial amount more, you could maybe consider Luxair next time, via Manchester I believe.