By your argument, Linux itself is a monolith and an evolutionary dead end.
Whether something is a big 400-pound gorilla or a swarm of ants matters much less, in the grand scheme of things, than you think. Your M$ argument is a straw man: one is closed source, the other is not, thus you're comparing apples with rocks.
I would surmise that the ease of adding new and important features (or just fixing bugs, particularly when they require nontrivial changes) is far more important. And that's precisely where having one common repository shines -- you can make one branch with your change, test it, merge it, you're done, instead of coordinating between N repositories and handling the resulting version mismatch.
NB: Please either answer my question, or admit that you can't.