Who else has the final say, then, if not the DM?
It has to be someone. Not the group, not consensus, but someONE; as - speaking from experience both in and out of RPGing - having a single final-word authority to fall back on when other means of resolution have failed is the only way these things can work and remain sustainable.
Who has the final say when spouses look for dinner? Who has the final say when four friends are deciding what to do on their twice-monthly get-together? Who has the final say in a group project with groups assigned by the instructor? There are
tons of situations in human organization where you do not have an automatic, single voice that lays down the law, but rather
work toward consensus or
agree that consensus cannot be achieved.
I completely agree that for
some things, a person who makes a judgment call is needed. That's when people disagree about
rules. And when I say "rules," I mean rules, rulings, houserules, vague recollections, ad-hoc band-aids, whatever it is that
adjudicates. But rules-adjudication is not what's happening when someone says, "I don't run games for evil PCs, because I doubt my ability to run an enjoyable game for them." That's about as far from
settling a dispute judicially as one can get; the person saying that, far from being a judge separate from the parties involved, is in fact one of the petitioners advocating. To call one of the petitioners "judge" is to be, quite literally,
prejudiced: to judge the case in advance. It is, instead, an expression of personal taste; far from being an adjudicator, the speaker is
advocating for their value-judgment(s).
The DM's role features a lot of adjudication, yes. But the DM is far more than a
mere adjudicator; that was one of the most common (and, for me, frustrating) criticisms levied at 4e, after all, that the DM
needs to be more than merely a central processing unit for the rules. Things like "I don't run games for evil PCs" are not firm and final answers about
rules, they are expressions of personal interest and taste. Those are things which
should be talked out. I have often said here
de gustibus non disputandum est, of taste there can be no dispute, but that is no refutation of this. Indeed, it is rather saying that one should not
conflate discussions of taste (where truth-values and correctness cannot be defined) with disputes about rules (that can be adjudicated, where truth-values and correctness are defined but not always obvious).
Both things, rules-adjudications and value-judgments, need to come to some kind of conclusion, even if that conclusion is "we cannot agree and must thus separate." But the process of coming to a conclusion for value-judgments generally is (and generally should be) very different from the one used for rules-adjudications. Since each person equally brings their own set of tastes and preferences, even if some exert more labor, every participant has an equal right to argue for their preferences, and an equal duty to respect the preferences of others (the two statements are equivalent, just phrased in terms of those who give, duties/responsibilities, or those who receive, rights.)
Hence why I categorically rejected the argument earlier that the DM is the one "responsible for everyone's fun."
Everyone is "responsible for everyone's fun."
Anyone can harm everyone's fun. Further, just as every duty implies some kind of right and vice-versa, everyone has the
right to have fun during the game (and thus to leave, if the game is found lacking). It is this reciprocal relationship which enables tabletop roleplay to be as enjoyable as it is.