You know, there have been times when Paladins couldn't smite, and at one point, one of their big class features was the ability to wield a holy sword. The exact mechanics used to express the "class fantasy" are always shifting. Over the decades, we've seen Paladins who are tanks, and Paladins who are offensive damage dealers. The pendulum swings to and fro.
The Monk, however, seems to have a problem shedding it's skin. It's central mechanics generally are as follows:
*ability to function without wearing armor in a game where melee fighters are intended to wear armor to protect themselves.
*ability to function without using weapons in a game where melee fighters are intended to use weapons to deal damage; alternately, sometimes there are mechanics to allow them to use inferior weapons more effectively (in a game where melee fighters are intended to use the best weapons for the job).
*a few largely unique defensive abilities, loosely based around protecting them from ranged attacks, magic, and detrimental effects.
*a grab bag of random abilities you'd expect from an "enlightened old master". Longevity, the ability to talk to all living things, and a lot of design space devoted to a cruddy version of a first-level spell (feather fall).
*the ability to go fast.
Monk mechanics are always fiddly and more complicated than what other classes do. I find this ironic, as most people who want psionics, for example, want psionics to have completely different mechanics from other spellcasters, and there's usually a lot of people who, if psionics have to be a thing, don't see the reason for a completely different magic system.
And yet, with Monks, we have a completely different combat system than that used by other warriors. Monks have a limited pool resource that powers all of their special combat maneuvers as part of their base class, which refreshes on a rest.
You can find mechanics like this in subclasses, like the superiority dice of the Battlemaster, but no other non-caster class works this way. The Fighter has several short rest abilities baseline, but you don't have to choose between, say, Action Surge and Second Wind, for example, nor are you restricted from doing both if you need to.
Subclasses that grant new abilities to another class that require the use of resources, have their own bespoke resources to use, again, the battlemasters gain superiority dice, arcane tricksters gain spells, etc.. But in the Monk's case, their subclasses say "hey, since you already have ki, we'll just give you more abilities that use it!".
Going back to psionics, even though I've often agreed with those who want psionics to feel different, the Monk is actually an excellent argument for why that would be bad for the game! The Monk class suffers greatly because so much of it's design is centered around trying to make it completely different from other combat classes! And, as I've noted before, the end result is rarely the best at anything!
The Rogue can be built to be a better skirmisher and is way better in the skills department. The Fighter can make more attacks for more damage. The Barbarian can fulfill the class fantasy of "guy in loincloth fending off dozens of foes" better than the Monk can. Melee control? A Battlemaster with weapon masteries can't stun, but is otherwise a serious contender (and if Cunning Strike remains, the Rogue is potentially even better!). Even the Monk's most unique feature, it's great speed, can be replicated by a 1st-level spell (and a 1st-level spell, as I pointed out, is superior to slow fall).
Whatever it is that the Monk is supposed to be, be it peerless melee combatant, skirmisher, or "annoying kung fu guy who can never be hit", they aren't really good at it.
Like the Ranger, the Monk also suffers from the designers not "picking a lane" for the Monk. Most of it's design is full of artifacts from previous editions; editions where it was mathematically proven to be a terrible class (and yet, at least once a week, a thread griping about how Monks were OP and/or people just didn't understand them would pop up on every gaming forum- a trend that continues to this very day!).
Monks are not good tanks. Being a tank isn't just being unhittable. You need to also have the ability to force enemies to focus on you, as opposed to other party members. Almost nobody is a good tank in this sense, but it's ok, every class except the Monk can be built to wear the best armor in the game if they really want to!
That's the reality of the current iteration of D&D. The Monk either needs to be rebuilt from the ground up, to fit in this paradigm, or die. There's no reason to put this class in the PHB except as a shining example of ivory tower game design.
