I think the reason they left out oath of vengeance is that they wanted to go with the two oaths that feel thematically similar so that iftheir wires got crossed due to oathbreaking, it wouldn't be too incongrous. Otherwise if they'd included vengeance paladins, you might get a situation where you can break an oath for being merciful as a devotion paladin, or break an oath for being rutheless as a vengeance paladin.
As for people being upset about Oathbreaker, I think the problem comes down to the oathbreaking mechanics kind of disconnecting them from their character. They do things that they, with their understanding, feel should be well within the boundaries of their oath, but suddenly it's not and they have to deal with it. I think that in a tabletop setting, that would be equally unsatisfying. It feels like losing control of the fate of your character. Whether that's accurate or not, that's the feeling and it's something beyond how mechanically good Oathbreaker is. I think the people complaining want to be morally upstanding paladins, but their idea of what that means clashes with the game, and now they have to interpret what the game will want from them. I ran a paladin after patch 9 but I stopped because it was genuinely stressful wondering what might or might not break my oath. And I never even became an oathbreaker. I sincerely advocate that things, especially choices that would break your oath should be marked as such because I think that outside of extraordinary circumstances, our character would know most things that would be against our oath, and that's knowledge we as players should also be privvy to.
