How to make brackets work like group separators?

Question

I know that the following is not something you are usually supposed to do, but I ask anyway: I am getting quite tired of the fact that TeX does not treat the content of brackets [...] as groups like {...}. This is getting annoying when writing math with lots of nested keyval interfaces, where stuff like

\command[key=\othercommand[otherkey=7]]

dies horribly. In other words, you have to wrap the key in braces:

\command[key={ \othercommand[otherkey=7] }]

I wonder: Does it have to be this way, or could you in principle make [...] behave like group separators (like {...}) and still make optional arguments like in xparse work? I am well aware that stuff like $[a,b)$ would not work and would have to be replaced by $\lbracket a,b)$, but I’m fine with that.

I tried something like the below, which obviously failed horribly:

\documentclass{article}
\usepackage{xparse}
\let\oldopenbracket=[
\let\oldclosebracket=]
\def\MakeBracketsStrict{
    \catcode[=13 \catcode]=13
\def[{\oldopenbracket\noexpand\bgroup}

\def]{\oldclosebracket\noexpand\egroup}

}
\def\MakeBracketsNormal{
    \catcode[=12 \catcode]=12
}
\begin{document}
\DeclareDocumentCommand\test{ mo }{we test #1 and #2}
\MakeBracketsStrict
\test{foo}[bar]
\MakeBracketsNormal
\test{foo}[bar]
\end{document}