Kensei being kinda sorta broken by circumventing the main restriction the class was built around is kinda proving the point.
The monk is built around one trope and any attempt to expand it mechanically results in something too weak or too strong.
The Monk shouldhave been designed with flexibility at the start. Doing it at the subclass level with such a rigid base is the problem.
I actually agree with the idea that in a different universe the monk would have been better designed to be more versatile with the base class, however the lack of that design principle doesn't mean the monk is a failed design.
How many different flavors of rogue can you make from the PHB base class options? It's pretty much picking the skill you want to be good at in addition to Stealth. How different would the various paladins feel without their subclasses (and even then I feel paladins of all subclasses play the same on the table). If there is a one-note problem to a class it's NOT the monk, who along with the bard is a very tool-to-fit-the-situation character.
Want them to fill in for a tank? They have Dodge as a bonus action.
Want them to be mobile? They ha e Dash as a bonus action.
Want them to kill something? They have an extra attack more than most characters at their level.
Want them to crowd control? Flurry of blows and multiple stunning strikes and you could conceivably drop a stun on 4 opponents in one turn.
You can even switch these styles up in the same combat on the fly.
Yes, doing these things cost ki, but costing ki is what makes the useful monk not be the broken monk. You get it all back on a shirt rest so as long as your GM keeps to expected rest options in their game you aren't going to be tapped out with nothing to contribute before the halfway point in the adventuring day (unless you choose to).
Also I haven't ever heard of the Kensi being out on the list of overpowered 5e options. It's basically a way to move your monk squarely into the fighter lite role and in that role it can perform well but isn't skewing an encounter the like paladins can when they oneshot a monster designed to battle an entire party or
the barbarian can by rage absorbing hundreds of extra HP of damage leaving the rest of the party unscathed.