$ ls -l /bin/bash /bin/dash /bin/mksh
-rwxr-xr-x. 1 root root 964536 Nov 22 2019 /bin/bash
-rwxr-xr-x. 1 root root 113536 Nov 5 2018 /bin/dash
-rwxr-xr-x. 1 root root 296192 Jan 27 2018 /bin/mksh
https://www.irif.fr/_media/rencontres/pps2018/regis-gianas.p...
I would say that mksh implements 80% of bash functionality in much less space, and closely follows ksh88; mksh is also licensed such that Android uses it as /bin/sh.
Apple has switched from bash to zsh. I don't know the motivations for this, but preferences for shells wax and wane. BSD doesn't include bash (in base) more because of license than code quality.
POSIX seems to be all that we can agree on, but I do wish that standard could grow; it's stuck in the '70s.
GPLv3 vs BSD or MIT license. I lived zsh anyway, switched some of my BSD shells to it afterwards.
I don't like the fact that you have no way to clear the history buffer if you've accidentally dropped a password at the prompt. Bash has history -c. I don't have shared history enabled, you could unset that and it'll stop segregating.
Anyway, zsh is a lot more versatile and familiar than tcsh for instance, so it's almost always my go to shell on FreeBSD as /bin/sh is quite limited, but also miles ahead of the Debian shell regarding features.
It's an interesting idea, and while I feel bash is absolutely 'good enough' to be part of the base system, I wouldn't want zsh or fish part of my base system - so it's then a matter of opinion whose shell is bloat and whose is essential. So I respect their decision to not include bash in the base image - it's meant to be uncompromisingly lean and simple.