I remember a while ago I was looking at what I considered the "classic" D&D bosses and mid/high and higher levels:
CR 13 - Beholder, Rakshasa, Storm Giant, Vampire
CR 14 - Ice Devil
CR 15 - Purple Worm, Vampire Spellcaster/Warrior
CR 16 - Iron Golem
CR 17 - Death Knight, Dragon Turtle
CR 18 - Demilich
CR 19 - Balor
CR 20 - Pit Fiend
CR 21 - Lich
and of course adult dragons (CR 13-17).
Some epic games (IME) have even gone against ancient dragons, devil/demon lords, demigods, or the Tarrasque (in prior editions or 5E).
Now, I was looking at this list and CR vs. party of PCs, and I realized that IMO to keep these bosses "deadly", PCs really need to be capped around 15th level or so (or even lower, maybe 12th or 13th for the lower CR bosses above). Only when you want to use the more "epic" opponents can you have PCs of higher levels, especially by tier 4.
A problem as I see it is few tables ever reach the epic/paragon/superhero tiers in play. My groups have only done it once, but part of that is also because I (as DM and player) have little interest in playing at that level of power. I would think more the people who want to play at that high of a level would more likely start higher, but I could easily be wrong.
Anyway, since few games reach that point, maybe WotC didn't do as good a job developing 5E martials for it. The casters already had the magic from prior editions, so that was easy enough to carry over, but in the process martials go sort of left behind. WotC did the work, testing it, and at the time I suppose maybe it seemed acceptable to the play testers and designers? Regardless, there are some exceptions sure, but by and large they are lackluster compared to the mighty magics beginning in tier 3 through 4.
If people want a superhero/epic tier 4 implementation, it really wouldn't matter to me or affect my 5E games. We won't reach that point most likely.
I will conclude by acknowledging our games have been using the Faster Feature variant I developed a long time ago. Our PCs reach their full feature list by level 15, but spells aren't accelerated (as I've mentioned before, we've rather nerfed spellcasting in general to keep magic "rarer" or "more special/magical" at our table) to match it, so spells lag a bit.