> * The syntax occupies what is, for me, an awkward middle ground: it's a bit better than POSIX syntax if POSIX shells didn't exist, but not enough better that I care to juggle the two syntaxes;

This. It's an awesome shell, but I couldn't set is as a default because my already done shellscripts and aliases would stop working. And I also can't set it on server, because then a deployment tool logs in, and expects bash.

And if it isn't default, I might not bother, I'd forget to run it manually most of the time.

(The worst offender is replacing && with 'and' - why not at least enable both and preserve compatibility?)