[okdana]

>Shells like zsh and fish will never get mainstream adoption because they are not compatible with bash.

zsh and fish don't really belong in the same comparison IMO.

zsh is, like bash, a ksh-like shell with a Bourne-style grammar. Obviously it depends on the exact use case, but in practice, for basic scripting purposes, it is almost a super-set of bash, and it provides several emulation options (like sh_word_split) specifically designed to increase compatibility with POSIX and with bash in particular. It even provides shims to support bash completion functions. (It is fair to point out that, even with all the emulation stuff enabled, it's still not completely bash-compatible, nor POSIX-compliant. It's close enough that the changes required are usually extremely trivial, though.)

fish on the other hand has its own completely different grammar and makes no attempt to provide POSIX/ksh/bash compatibility at all.