IMO, they are supposed to be skirmishers and assassins. They get lots of movement buffs.
So running past the front line to punch the wizard in the back and get him to drop concentration, or getting in the archers face to give them disadvantage. Not standing still trading blows with a guy in armor and a great sword.
Could probably emphasize it a bit more at level 1.
Evasive footwork: "when you take the dash action, you do not provoke opportunity attacks until the end of your turn."
Now at level 2, you can spend a bonus action + ki, jump over the front line and kick an archer in the back.
Something no other class can do. (Except the occasional misty step).
Except a rogue, who can do basically the same thing without having to use the ki.
No not quite, look at the math. The rogue is easy, it deals 1d8+5+[1/2level]d6. A sizable amount of damage yes. The monk is no slouch there though & without using resources other than action & bonus action it does damage that is fairly similar to the rogue all day through all of the 6-8 encounters the GM is expected to throw out.
mellored's example of punching a wizard in back to make him drop concentration is where things start changing dramatically though. The rogue with cunning action needs to force a failed save but the monk with +10 no action cost speed has two options. The choice is simply to declare "and I'm going to make that a stunning strike" on any or even all of those 2-4 attacks that succeed to trigger a con save or stun where concentration continues but the caster can't action/bonus action cast more spells or trigger the ongoing effect as applicable to the spell being concentrated on. In actual play with 5e spells the value of ensuring the caster won't cast another spell & granting everyone advantage on attacks against them is almost always dramatically more useful than ending concentration. If you replace "caster with ogre troll firegiant or whatever the same complete nullification from the monk holds true"
Yes at low levels Ki is fairly limited but in late tier2 & into tier3+ that's not so true. My current group just hit level 10 & with an assumed 2 short rests most adventuring days the monk in the party has up to 30 or more points of Ki every adventuring day giving it plenty of gas in the tank to nullify any weighty elite type monsters they face. If by chance all of the monsters are elite types that might tax the monk's Ki then the group has a good case to force a short rest either by refusing to continue or pulling back to rest a bit.