I agree with the sentiment. But I didn't want to be too critical about it, because like you said, it's their project and they don't have to give particularly strong reasons for what they choose to spend their time on.

And after all, this approach is probably fitting for a non-POSIX compliant shell like fish. I expect that those who would rather break compatibility in exchange for a "look & feel" that appears more "modern" are likely to also prefer breaking support for some old but stable tooling by switching to a more "modern" language ecosystem, even if doesn't have the battle-tested history and wide compatibility of more traditional languages. Those who care about that kind of stability might be better off using more traditional shells anyway. So maybe doing this would actually make working on fish more attractive for the kind of devs who would be more likely to contribute. We'll see.