Did you also notice, that after people are released from UFC (but they are still active fighters), their career flies down ?
This is not true and I can give you few examples, Segio Pettis is a good MMA example, Cris Cyborg is doing great in Bellator and boxing,
Ryan Bader is great in Bellator as Heavyweight champion, Liz Carmouche is Flyweight champion in Bellator.
Francis Ngannou is just getting started.
All of them are doing great against "non ufc fighters". Some people say that this is similar to going from professional to amateur, of course you gonna do great there. Like NHL players more to their local leagues and play well there; become champions of Sweden League B for example. But I've got your point. I am not praising UFC, not saying that their fighters are cool and others suck. But in other promotions former ufc fighters imho act like a promotion bonus. Promotions can speculate with "look at this guy, he is from UFC, you should not miss that fight, buy our tickets" and etc.