Flex Mythic would likely be more toxic for the endgame raid scene than anything Blizzard has tried thus far. We'd just end up with guilds benching 2/3rds of their roster for progression kills to get whatever "magic number" makes encounters easiest.
The 10/25M dichotomy was attempted for two years but I don't think it ever lived up to the vision Blizzard had for endgame raiding and created innumerable tuning issues on top of effectively restricting the devs to design encounters for two separate demographics.
There's an argument to be made that 20M was the wrong size to settle on after providing players a logistically easier 10M raid option for two expansions. But it stands to reason in a game with as many classes/specs as WoW currently has, it was an intentional choice to ensure diverse representation.
The common magic number I see thrown about is dropping Mythic to 15M. I could see this potentially happening in the future if the game's playerbase shrinks considerably but from the dev interviews it does seem as if they're happy with the way things are at this point in time. As @
Osmeric has mentioned, I think it's more likely that by the time the game reaches such a point we might simply see an end to Mythic raiding altogether. Time will tell.