The general concensus is that Mythic raid balance thus far has been a complete failure, with Mythic EN being grossly undertuned and Mythic ToV being overtuned. Mythic raid balance has arguably gotten worse since the the switch to a fixed raid size of 20.
Yet despite these tuning woes, these very people are still in support of a fixed raid size of 20. Why? If these raids are anything to go by, it proves that major balance issues would still plague Mythic raiding despite having a fixed raid size. Which was the major argument for switching to 20 in the first place. It proves that raid size was not the major contributor to raid imbalance.
I think that the Mythic raid size should be made more flexible to allow more players to access the Mythic content. They can start with two raid sizes, 16 (2/3-4/10-11) and 20 (2/4-5/13-14), and go from there with 12 following shortly after (2/2-3/7-8). Sure, they may not be perfectly balanced, but as seen so far, things will still be imbalanced no matter if the raid size is fixed or more flexible. At least this way more people will get to enjoy the content. Accessibility > balance (which is impossible to achieve anyway).