I think you’re confusing rationality for plain self-interest. If you have something to protect, then it may be reasonable to sacrifice personal comfort or even your life to protect it.
Also, you comment implies that the only reason you’d fight for something other than yourself is out of “liking the idea of yourself as some sort of social justice warrior,” as opposed to caring about something and believing you can win by applying some strategy. And saying you’d “ruin your life” implies a set of values by which a life would count as “ruined.”
Yes. In many real-life scenarios, this is true. In small games where the rules are blatant, it’s easier to tell if someone is breaking an agreement or trying to subvert you, so model games aren’t necessarily indicative of real-world conditions. For a real life example, look at the US’s decision to fund religious groups to fight communists in the middle east. If someone wants to destroy you, during the alliance they’ll work secretly to subvert you, and after the alliance is over, they’ll use whatever new powers they have gained to try to destroy you.
People make compromises that sacrifice things intrinsic to their stated beliefs when they believe it is inevitable they’ll lose — by making the “best bet” they were revealing that they weren’t trying to win, that they’ve utterly given up on winning. The point of anarchy is that there is no king. For an anarchist to be a kingmaker is for an anarchist to give up on anarchy.
And from a moral standpoint, what about the situation where someone is asked to work with a rapist, pedophile, or serial killer? We’re talking about heinous beliefs/actions here, things that would make someone a monster, not mundane “this person uses ruby and I use python,” disagreements. What if working with a {rapist,pedo,serial killer} means they live to injure and kill another day? If that’s the outcome, by working with them you’re enabling that outcome by enabling them.