This reminds me of this SMBC. There are fields (modern physics comes to mind too) that no one outside of them can understand what they are doing anymore, yet that appear to have remained sane. There are more safeguards against postmodernists' failure mode than this one. In fact, I think there is a lot more wrong with postmodernism than that they don't have to justify themselves to outsiders. Math and physics have mechanisms determining what ideas within them get accepted that imbue them with their sanity. In math, there are proofs. In physics, there are experiments.

If something like this safeguard is going to work for us, our mechanism that determines what ideas spread among us needs to reflect something good, so that producing the kind of idea that passes that filter makes our community worthwhile. This can be broken into two subgoals: making sure the kinds of questions we're asking are worthwhile, that we are searching for the right kind of thing, and making sure that our acceptance criterion is a good one. (There's also something that modern physics may or may not have for much longer, which is "Can progress be made toward the thing you're looking for").