The problem with reducing the size of a raid is that, once you're down to 10, it's very difficult to balance stuff. With 20, you can tune stuff with the expectation that there's one of every class, one of every buff, plus competent players. If you're at 10, that's no longer true: there are 11 classes out there (soon 12), so one of them has to sit. Then, it becomes difficult to juggle stuff around, meaning the composition of raids become much more static. And that's a death spiral on the interest of raids as you need to allow more flex, which reduces difficulty, which reduces prestige, etc. Before long, Mythic has no meaning.
So yea. 20-man is probably the best format for Mythic-tuned raiding.
Alas. Maybe.