It's not a matter of competing and more that magic items are replacing mundane class feature.A fighter is perfectly complete without a single magic item as far as I'm concerned. If the DM is not providing magical weapons and still throwing monsters that are immune to magic that must be defeated? No different than a DM regularly throwing wizards into anti-magic zones.
A martial has no issue fighting a devil.
But a fighter shouldn't need to receive boot of speed to run faster and boots of striding and springing to jump farther. A normal human so trains physically for years would do so normally.
Magic items was a kludge to keep martials simple. And we just got used to it.
Much of the supernatural martial push is just the expansion of stapling those magic items to class features and then people scaling it past human limits.
Like I said way in the beginning, a high level mundane martial is possible. It just requires making a bunch of the weaker magic items class features and thus expanding the turns of martials to those of casters or even longer.