I am new around here, but may I ask why distributing the GNU Bash
   shell hasn't been considered?

It *has* been, and was quickly discarded.

1) It's buggy, WAY too large, and WAY too slow.

2) It's GPLed.

3) It doesn't have arrays.

   It can't be strictly because it is GPL'd
   code because gcc is in netbsd [...]

Your reasoning is flawed.  GCC is a critical component for which no
reasonable alternative exists.  ash is a critical component for which
many alternatives exist, bash being only one of them.

   (albeit an old version).

An old version that *works*, I might add.

   [...], and bash is a completely POSIXized shell.

That is only true if a configuration option was added to remove all
non-POSIX builtins and shell variables by default.  Last I knew there
was no such option.