It's not a failure of game design.
The designers and community don't know what high level is to design for it.
The D&D community for 50 years demanded that gaming level D&D existed but you 40 of those years only had a concept of what it is on the monster side.
Again...
We all agree on the Archmage, High Patriarch, Ancient Dragons, and Lich.
But there is a void creates 20 concepts of the what the high level version of a Thief, Berserker, or Champion.
Is a level 20 Drunken Master:
Lei Wulong
Jackie Chan
or
Master Roshi
Because the 3rd guy casually blows up the moon to stop lycanthropy.
problem with high level maritals is, how magical do you want them to become?
If you want them to stay completely mundane, then only solution to stay somewhat close to casters is to buff their damage, number of attacks, attack riders(mundane), some skills, more expertise, more AC, maybe 2 reactions per round for AoOs,
now, is it enough that your clumsy 1st level archer that can barely shoot 1 arrow per round becomes Lars Andersen and shoots 15+ arrows per round?
That is impressive, but if that is all he can do, it can get very boring very fast.
but some people really want to roll 15+ attack rolls every round. So, it's fun for them.
on the other hand, we have the Book of nine Swords.
do we want martials to become that amount of magical?
also, most campaigns are done by levels 10-13.
we need ALL the goodies for classes(except spells) to be "on line" by that time. At least in some basic form.
Upgrades can be for high levels.
I.E.battlemasters "unlimited" maneuvers with Relentless should be at 10th level latest and not 15th, that is too little, too late.
maybe even 7th level with d4 dice to power them. d6 at 10th level, d8 at 15th level.