I get your point tbh, it always seems like they release someone and then they either fall into obscurity or lose in other promotions. However, it's rare that I think it was an unwarranted release. The last time people were up in arms about a release was when they released (or didn't re-sign) Overeem and JDS.
But would you really be surprised if Overeem and JDS fought in another promotion and lost at this point in their careers? It's shocking because they're huge names in the sport but people need to take a step back and look at where they're at in their careers. The UFC looks cool when they do it because people are too focused on the name... that's Alistair Overeem, that's Junior dos Santos, that's Anthony Pettis... you're really gonna release them/let them go?
In hindsight, Collard beating Pettis wasn't really surprising. Collard's improved a lot since his UFC stint and has actually made a small name for himself in boxing beating a few undefeated fighters. He's fighting a past prime Pettis, who's always been a striker. Pettis is the name in MMA so he's expected to win, but going into that fight, you would be crazy to think it would be an easy win for him. Collard can probably box up plenty of guys in the UFC right now. We'll see how he does against a PFL lay n pray-er though.
Most of the time, they release guys who are old or past their primes. So it's to be expected that they lose and the UFC's decision in releasing them is validated. Jon Fitch is a good example though. Similar to Pettis, his first fight outside of the UFC in PFL (WSOF at the time), he lost. He still ended up finding success outside of the UFC and didn't fall into obscurity, I think that mostly applies to lower-level guys. But people were outraged at his release at the time because he was still a top guy, but... is the UFC really missing out on a lot in releasing him? No. One fighter isn't going to make or break the UFC. They released Fitch because of his boring style and you could see a pattern in the fighting styles of the guys they release. Exciting guys get more chances, grapplers have a shorter leash. Fitch happened to lose when they let him go, so that may be the best example of what you're talking about when it comes to the UFC being "right" when everyone was outraged and it seemed like the wrong decision.
I think another important point to bring up is the skill gap isn't as large as people think between fighters in the UFC and other promotions, and that also could be a reason why freshly released UFC fighters lose when they fight elsewhere. Of course, the UFC has the best fighters in the world and it's not debatable. But plenty of fighters in other promotions could win fights in the UFC. If you think a random Contender Series guy is better than someone like Adriano Moraes (who recently beat Mighty Mouse in ONE) or some random Russian on the next ACA card just because that Contender Series guy is in the UFC, then you're just plain dumb. We're way past that point when it was very clear that UFC fighters were vastly superior to fighters in other promotions. Keep in mind, we're not talking about the tip top fighters, because these guys typically don't get released. Yoel Romero was released, he's in his 40s and coming off a 3 fight losing streak... but these losses are to arguably the top 3 fighters in his division. So he's still a top fighter, right? Well he's been losing, so we ultimately don't know what level he's at right now. Is he still a top guy? Would it really be surprising if he lost in Bellator when he's fighting in the one Bellator division that some people argue could rival the UFCs? He's 44 years old. It's not like the UFC is releasing top guys that have a high upside. Fitch was coming off the worst stretch of his UFC career when they released him as well. So they are "good" at releasing guys at the right time, but it's also not a very challenging task when there are plenty of hints and these decisions essentially don't impact their business much at all.
