Note: While I don't think people in Worm would necessarily take anything at face value, I also don't think any of the Masters would reject the rules of the War out of hand either. Given how "similar" powers like that of Teacher work, it is perfectly reasonable for people to assume that they really couldn't maintain their Servant indefinitely without the Grail. The more worrisome question would be about whether the Grail can even grant wishes, but at this point we don't really have a Master that would be overly concerned by that question. Blasto, as someone pointed out, would fight if only to keep his new waifu, and Burnscar is definitely unhinged enough to believe that. Armsmaster, Vista, Theo and Bakuda participate because the War itself is a means to an end and because naturally the actions of one another, from Theo killing E88 members to Bakuda's everything, bring each other into conflict. Alec is a bit weirder. He wants to learn more about himself and hopefully remove whatever vestiges of his family's influence once and for all, and at the moment he hasn't been set on a path that directly intersects with anyone else (I'll try to rectify that soon). I do think, however, that Alec (and a lot of the Undersiders for that matter) has quite the skill for self delusion, especially when it comes to ignoring problems that would weigh too heavily on their minds (canon Taylor is the queen of this), so I do think that Alec would do quite well at not thinking to hard about the plausibility of the Grail's claims.
