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.