I agree that might be a good choice for a more modern alternative, but it still doesn't solve the problem of existing manpages. You could cleverly upconvert on the fly of course, but that's basically the same thing as keeping groff around, except with the added friction of informing everyone that they should be using Markdown now, with new tooling and conventions. Don't underestimate the marketing costs of successfully convincing the diverse Linux ecosystems they should jump onboard.
I only see this working if some big entity slams the gavel and decrees this as the new manpage standard, but I also don't see any such entity having much incentive to do so.
Well... manpages are still very much alive and widely used, and they're so prevalent that moving to something new is a nearly ridiculous requirement. Half the things with manpages in the wild are probably themselves not maintained. Now you have two problems.
We can move to something new, but getting everyone to agree on what that is, then implementing and maintaining it... isn't that more work than just maintaining the working solution everyone already agrees on, and that solves the problem?
I'd limit it to FOSS that people don't pay for (directly or indirectly).
When money is changing hands with the expectation that software is maintained in good working order, you won't see abandonment like this. But when nobody is paying or getting paid, maintenance stops, regardless of whether the software is a key component of crypto on the internet or a fundamental part of the documentation of most Linux installations.
Remember that the worst bugs in the last couple of years have been in the code that nobody was looking at.