I'm going to write this in between league games queues, so thoughts are probably gonna be scattered.
Vote manipulators(at least in the current meta, if you wanna call it that, and most games) are necessary. Otherwise nothing really happens to cause conflicts except murder, but that's a staple in most games. No conflict causes games to become stale and kind of repetitive and pointless, and doesn't leave much room for character development. I'd like to point at Nelly's most recent game here as to why conflict is necessary. Nobody betrayed, and nobody died(although there was a very shitty attempt at murder). I think the game had potential to turn out much better than it did(not saying it was bad), but towards the end of the game everything felt pointless. There was no reason to be scared or whatever. It was just a "wow we're here" to "wow we're out." Everyone just allied. Nobody tried to kill.
It's like you said, that often times the benefits of betraying are not worth it. Take a look at, say, what happened to Munch in UaS or Arc's ongoing game(illegal but I'm not scouring games). Munch was immediately attacked in UaS and in the recent one he was ganged up on for betraying. Now, in UaS, he literally couldn't die conventionally, but he sure as hell would have been. The point is, people react violently to betrayal when the main point of it should be survival(in character) - why do I want to stay in here and prolong my chance of death, instead of saying "FUCK YOU" and getting out of here because I fear for my life? While you do that in character, you OOC know that you will be killed. And some players will create IC reasons to kill you for that reason.
The prior incentive to earn more BP is essentially nullified as there's no way to take the blame off of you, the only one to blame is your partner, who will easily be working against you now. This isn't even speaking of 9 player games where one side of the AB Pairings is a single player, so there is absolutely no one to place the blame on. Even the idea of defending against an expected Betray is dependent on the other players around, and it's likely that a lot of those players will see this as an opportunity to exploit and manipulate all of the attention and hatred towards one/two players instead of themselves.
Voting pairs will be further manipulated now to prevent you from earning more points and to catch up the players betrayed against, or you will be killed to prevent any more hampering to the point gain of all the other players (i.e "Kill all Betrayers" mentality).
Voting pairs are usually taken care of by the host with randomized groups that are selected from there(in Nelly's game there was the pick the group option, but why use it when there's no threat?). I think you're also missing out on a huge problem of betrays - to make use of it, you need to betray twice in a row. Sure, you can use betray defensively, but then you're sitting at 11 points and pissed off at LEAST one person. Explain yourself, do whatever, you're probably going to die. It's hard to explain when the person you suspect also didn't vote betray previously. Basically, there's no point to vote betray after the first round, where you're likely unable to defend yourself, and betrayal after that only draws immeasurable amounts of pissed off people towards you, who probably have arms at this point or are killers looking for a casus beli. Back to the main point, who is going to be STUPID enough to let you betray them after you've betrayed once? This then leads to the fact that most players have common sense, and to likely be able to get another betray off, you need to murder TWO opponents you're up against, while somehow keeping your name clean. Say you betray again - well now you can escape, but the other 11 people who don't have 12 are going to try their hardest to make sure you don't escape. This comes down partially to a player problem of not realizing how hard it is to kill a human for most people, but that's not the point of the discussion and we'll just talk in circles about proper roleplay. In short, the requirements to even make a betray work in face of all the risks are too damn high(hehe).
You can try to change betrayals to give other bonuses, but unless these go in the way of defending yourself versus, say, 3 people, how do you balance that, or explain it in story without complications? Maybe a +1 buff to each stats per person you are fighting past one? Then it's temporary, and they just wait. And then there's the problem of, even if you do have +3 STR, what are your odds of taking on three people? Really slim. You need some good damn rolls. Which is why buffing options for betrays is hard - hard to balance without it being meaningless, but if you make it too strong the game becomes a betray palooza. Allowing extra points to be found(like Nelly did in the create a role game) is a good solution to patch the betray twice problem, but then at some point you have people saying "wait why tf did he escape at 11?" It's certainly fair, but players just aren't happy unless they get their ending. Still, a 100% viable solution depending on how much BP is injected into the game.
I tried doing something like that in UaS with the greater chance to die the later you escape. Traitors escape earlier and have a lesser chance to die, and waiting for full ally makes it a cointoss whether you win after that or not. That still has the problem with betrayers getting attacked, but I think it's a good incentive to promote betrayers by jeopardizing the wins of those who just ally. I won't lie, vote manipulators are really fucking fun to watch the ensuing chaos and can be a good killer substitute, but it does get to the point where it's bastard and fucks people up for trying to play right.
TL DR Players ganging up on betrayers has always happened, but happened less frequently as games go on. Offering more incentives to betraying is risky as they can either be useless or too strong, hard to gauge from host perspective. The best ways are to inject artificial BP in the game, or jeopardize wins a la UaS to people who just infinite ally as to promote a more selfish playstyle.