Yep, most of the 130+ teams season's end with their first loss according to that logic According to that logic their seasons are over and the rest of the games are exhibition games.
By that logic it could be said that all the games are exhibition games. Any player that plays in any further game after they are logically out of the chase for the MNC would be playing in an "exhibition game". But in reality the whole season for any team is an "exhibition". The players don't make significant money by playing more games, even into their bowl game or playoff game for playing in those games. So why play in them, other than to put on an exhibition of their talents and further audition for the next level.
Oh, they play it for their team mates and/or for their fans. If they were really doing that they'd finish the season out and play in every game they can play in to finish up their careers with their team mates and in front of their fans.
The reason they opt out is because they believe they have already participated in enough exhibition games for the NFL scouts and teams and they have no further need for more exhibitions/auditions, so why risk injury.
Yes, opting out, judging when players have put on enough exhibitions of their talent, is a part of the degredation of college football.