I get the sentiment, but not sure I agree with the "either interpret it, or don't" part - it sounds like you are essentially saying the scripting should either match Bash completely (and thus lose out on the more modern syntax) or not offer any kind of helpful error to users. Or perhaps support both types of syntax?

Personally I think it's fine to help guide new users to the new syntax without directly supporting it - this keeps Fish scripts from all devolving into the (subjectively ugly) bash format. It's like a graceful deprecation warning.

Edit: although I do think ceding on the logical operators (&&, ||) was a good move since that one is so fundamental