Surely they the French could figure out that "neuf-dix-neuf" could work as well. I'm learning some South East Asian languages and the simplicity is astounding. Who needs "dix" when "dua dua" works just as well? (Indonesia)
My uneducated guess is that at the time, it was easier to keep the base 20 to visualize a higher number. Just like we say "a few dozen eggs" to get an overall sense of the volume, it made more sense to keep it that way instead of a more arithmetic approach. Not sure why it never evolved, tho.
Logical according to whom? Why is base 10 more logical than any of the other bases? I'd argue it's not, it's simply that it's what we're used to so we think it's more logical.
But I was more going on to the fact that 'base 10' is somehow more logical than any other base as a chosen one for natural languages. I see no reason why that is so.
The names of days are the equivalent of Day 1, Day 2 etc. (except Sunday which is Sunday/Skyday), and months are Month 1, Month 2 etc.