I don't really see it, tbh. You can tank a bit, but you don't have the hit points to take a huge hit, your AC isn't going to necessarily be as high as other classes and to-hit bonuses for enemies is generally enough to mean that even with Disadvantage you can still take hits.
At high levels your AC is better than most other martials. It tops out at AC 20, the same as a sword and board Paladin or fighter and they aren't dodging while they are attacking. At high level a dodging Monk is essentially a sword and board fighter with a permanent shield spell on herself who can also stun opponents regularly.
You won't take a huge hit when dodging, you will almost never take a critical hit and in addition to having good hit points, you have the ability to heal in combat while still attacking (although it is somewhat costly).
you take away Flurry of Blows,
FOB is usually a waste of Ki.
which allows for Stunning Strike.
Not very often in play. This is highly situational and rare. Keep in mind to use FOB you need to use it
"immediately after you take the attack action", meaning you can not move between attacks like you can with regular martial arts. So that means all enemies you want to stun with FOB need to be in reach when you attack.
Presumably if you are going to use stunning strike on your FOB, you are going to use it on your other attacks too. So for FOB to allow for stunning strike you either need 4 enemies within reach when you attack or you need a number of them to make a save such that you need that 4th attack to land a stun and you need to make this decision before you make your 3rd attack. This is not common at all, especially when you consider you are making the decision before the 3rd attack and you have to both hit and have the bad guy fail his save. Additionally if you have 4 enemies in melee reach it makes Patient Defense a much more powerful mathematically.
The fact you make this decision before the 3rd attack of the turn really complicates it as well. For example against one BBEG that you really want to stun - you get 2 attacks and 2 opportunities. IF both of those fail, well you still want to stun him, so at this point it is
"do I try with the 3rd attack or do I FOB to get 2 more" - if you FOB here are the possibilities:
1. You use FOB and you hit with your 3rd attack and land a stun - FOB was irrelevant (to stunning enemy)
2. You use FOB and you don't stun him on your 3rd attack (miss or he saves) and you miss with 4th attack - FOB irrelevant
3. You use FOB and you don't stun him on your 3rd attack and you hit with 4th attack but he saves - FOB irrelevant
4. You use FOB and you don't stun him on your 3rd attack but do hit with 4th and he does save - This is the only time FOB did what you are talking about.
That is after you already failed to stun him twice with your attack action.
It is just a very rare situation that FOB will make a difference in stunning an enemy. What is more common is using it to get a better chance of killing a near dead enemy by giving you 2 more chances to hit instead of 1, but there too it often goes to waste as you hit with the first attack pretty often. It also can be relatively good at draining legendary actions to set up a caster, but that is rare too because you an enemy with a low Con save to be able to get 3 failures in 4 attacks.
I played a Monk end to end from 1st to 20th level and above 15th level she was the most powerful martial at the table. I used FOB very rarely, I think about twice the whole campaign. I used Patient Defense all the time.
all the Open Fist powers, the Shadow Step, etc. You're making a very active choice to tank rather than just doing it,
To be clear you are making an active choice to dodge, not necessarily to tank. Dodging is a great improvement to AC, mobility and to dexterity saves, whether you are actively tanking or not.