Well to be fair, ENWorld is hardly representative of all D&D players. In a more general poll, I think it's likely a majority would be fine with the fighter as is.
My hunch is that the majority of players, especially the large swath of causual players would BOTH
1) be fine with the fighter as is
2) be fine with a mythic martial as an additional class option along side the fighter
There might be a large gravitation toward the mythic martial by casual players as I don't see them having a heart attack when presented with a concept that is pretty common place in fantasy fiction, movies, comics, etc. and by design the mythic martial will be closer to full spellcasters in power, versatility, and narrative control. But IMO the game would be better having both and letting the chips settle where they will.
It seems the only argument being presented that isn't a form of preferential gatekeeping is the resource argument -- developing and playtesting a mythic martial is a decent amount of work that could be used to produce more adventures, options for existing classes, etc.