The Bourne shell probably doesn't deal with reparented processes properly (at least, that's my experience with working on Docker (PID 1 is a weird process). Not to mention that your shell doesn't propogate signals to children so you'd need quite a bit of shell scripting to make it act like a real init system.
The main reason bash is the fallback init is because it's a good way to fix a borked setup. And "real men don't use init, they just need a root shell" (Linus).
The main reason bash is the fallback init is because it's a good way to fix a borked setup. And "real men don't use init, they just need a root shell" (Linus).