"MM" is even worse than "mm", because "M" is the international standard abbreviation for 10^6, so MM would by extension mean 10^12.

Except it doesn't. The metric system doesn't work that way.

M is not the international standard abbreviation for 10^6. M is the SI system for 10^6, which is not the same thing - SI has some extra rules.

If you write MM you are clearly not using SI (since it makes no sense in SI), so it's not that hard to figure out.

M has meant 1000 for a lot longer than it's meant 10^6.

And BTW, lowercase m means milli, so by your logic mm should mean 1/1000000 - so tell me again why MM is worse?

I meant to downvote but misclicked, so I'll just comment ars's comment above is to the best of my knowledge much more accurate than yours.