It may be a quagmire, but it's probably the only argument that actually has logical weight. If martials are so badass in combat that casters can't touch them, can't even come close ever, then there is a legitimate argument that martials shouldn't get an upgrade.
As an argument it beats the heck out of:
"Well that kind of martial just isn't D&D to me..don't ask me why or for any additional detail whatsoever. I either dont have reasons or cant be bothered to provide them"
And...
"Think of the newbies and lazy players..if you make it too complicated, it'll melt their poor wittle brains"
And...
"You whining brats just want magical superpowers that will break the game"
And..
"Well if you can't totally agree on the specifics of the exact changes you want it than you must not want any change"
And..
"Just use skills man, they're like the same as spells"
And..
"You already have mythic martials, just use the casting subclasses."
And...my personal favorite..
"We're devoting all this time and energy discussing this game design topic I brought up in this thread I created, when we should be out solving all the social issues in the hobby."
It's kind of refreshing to discuss something that actually makes sense from a game design perspective.