If we're taking that assumption we have to also assume no mythic martial. The same fundamental reason mages aren't going to get reigned in is the same one that keeps Martials from going in the same direction.
I did after all predict perfectly that they were going to buff Wizards, and the most we're likely to get is just a non-committal to the buff.
This was an exercise to show that push back to adding a mythic martial under those assumptions/parameters was either unwarranted or uncharitable.
Even though under these assumptions I would predict adding a mythic martial class would actually grow the audience / make the audience as a whole more satisfied:
People that think there is a disparity -- much more satisfied, some will play that didn't
Causal gamers or happy with the way it is but don't care about mythic martial existing one way or another -- not much change to numbers. probably casual a little more satisfied as this is a archetype they probably expect to be in there anyway. certainly won't turn anyone off.
Anti- having a new mythic martials as a separate class and in a splat book while maintaining all existing classes -- unhappy for unwarranted or uncharitable reasons apparently (see assumptions and reasons given)? predict many will get over it since their mundane martial classes remain in the game and they can play the same game as before if they want, but could be wrong?
It's impossible to read into the designers minds but something is going on. Maybe they like the disparity/gap and think it's part of D&D's identity? Maybe they think putting in the mythic martial will drive a huge portion of people away? Maybe they don't care about high level at all since most people play low level, not realizing the broken high level makes this so? Maybe they don't have the resources to do a full revamp or mythic martial well right now even if it's on the idea board? Maybe since table conventions make it work good enough for many tables, then good enough, ehh? Who knows?
But adding one additional Mythic Martial class that is no more powerful or versatile than the Wizard is not some system shattering thing. It points more toward wanting the aesthetics and disparity to remain.
But things change. Things go in and out of fashion. Executives get fired and teams get replaced, etc. Never say never!