Btw, if you ever wondered why Debian uses dash as /bin/sh (the switch was a bit annoying at the time), I think the reasoning was something like this:
dash is a bit faster, which might have saved a second or two on boot times (this was before systemd). Same applies to compilation times, configure scripts run faster with dash.
A bunch of #!/bin/sh scripts in Debian did not actually work if you replaced /bin/sh with another shell, which I guess some people wanted to do. Making dash the default /bin/sh forced everyone to fix their scripts.
Also some history on the abomination that is m4sh, famously used by GNU autoconf configure.ac scripts. Apparently when autoconf was released in 1991, there were still some Unix systems that shipped some 70s shells as the default /bin/sh. These shells do not support shell functions, which makes creating any sort of shell programming library pretty much impossible (I guess you could make a folder full of scripts instead of functions). They decided to use m4 preprocessor macros instead, as a sort of poor man’s replacement for functions.
In hindsight, it wish they had told commercial Unix sysadmins to install a proper /bin/sh or gtfo. But the GNU people thought it was important to make it as easy as possible to install free software even on commercial Unices.