It doesn't get rid of mobility because you can still bonus action dash or even dash and disengage. It reduces your speed, but you are still the most mobile class in the game even without the speed boost.
Um ... so you still have some ribbons, but you are losing the base speed.
When you are wearing armor, you basically lose all unarmed strike capacity.
You lose your bonus strike.
You lose your martial arts die.
You lose the ability to substitute dexterity for strength (unarmed and monk weapons*).
You lose the ability to sub in dex for grapple and shove.
*Monk weapons are simple melee or martial melee with light. Don't forget that monks have no weapon mastery, so if you're going weapons, that's a multiclass. And the monk now needs dexterity, wisdom, constitution, and likely strength.
Right there? That sucks. Who is going to use unarmed strike now? Can't do it as a bonus (flurry of blows). Does no damage. And if you want to use a strength weapon, that's another ability.
Next, you still have stunning strike, but that only activates with unarmed strikes (not doing any damage, and no bonus attack) and monk weapons.
You lose empowered strikes, because you're not using unarmed strikes.
You lose acrobatic movement.
You lose heightened flurry of blow, because you can't flurry of blows.
Also? A lot of subclass abilities are keyed on abilities you have lost. Shadow Flurry. Empowered Strikes.
But take the Open Hand (PLEASE!).
You lose all the level three abilities. You basically lose level 11. And level 17.
All of this is assuming, of course, that you retain the abilities that advance with the martial arts die. Because getting the martial arts die? Requires no armor.
Again, people should do the things that make them happy. But this seems a lot like playing a wizard without spells. Sure, you can do it, but why?