Mythic is not flexible due to the world first race. Imagine; Method beat Mythic Antorus with 14 people, Exorsus with 20. Cue endless, unanswerable debates surrounding who had it easier. For a race to have meaning the playing field has to be level. Hence mythic is fixed at 20. It's that simple.
On topic; I preferred 10 man raids myself, but with normal/heroic raids able to scale there is literally no reason to ever re-implement fixed 10 man raiding. Mythic plus also exacerbates this; Blizzard consider smaller PvE groups catered for, and they're right.