They can BEAT the damage curve.
They can BEAT the defense curve.
*while ki lasts.
And their niche is that they can run past the entire front line and stun the wizard in the back. They have a much stronger identity than most other classes (i.e. Rangers).
"Their numbers are a little bit low" is probably a valid complaint. But that's not inherent to the class design. Just increasing ki pool by 2 might fix that.
The point to which you were responding was specifically discussing monks at levels 1-4. So what is that niche, again?
Also, at levels 1-4 they can beat the damage curve, barely, for a maximum of 4 rounds at level 4. Compare a way of open hand monk to a berserker barbarian (off the top of my head) who can do two reckless attacks with rage bonus for the whole battle. Let's make it level 4 to give the monk maximum benefit. New monk does 1d8+2d6+9, for 20.5 damage; barbarian does 4d6+10 for 24 damage, setting aside weapon mastery effects. Anyhow, I think monk using flurry of blows is good, but not off the charts or anything. Fairly competitive with a raging barbarian. For four rounds.
Survivability...well, let's compare to that same barbarian, against, say, an orc. We'll just use unarmored defence for both, so the barbarian will likely have AC 14 with 41 HP, and the monk AC 16, 27 HP. Barbarian is raging, monk is using patient defence. And the barbarian is going to keep making those reckless attacks, so the orc will have no problem hitting. Orc does 4.17 HP damage/round to the barbarian (damage halved due to rage) and 2.13 damage/round to the monk. At the end of that 4 rounds, the barbarian will have lost about 40% of their HP, and the monk only 30% of theirs. So again fairly competitive with a raging, reckless barbarian. For four rounds.
But the barbarian is doing both at the same time - they have that survivability while still doing better offence than the monk (way better, actually, once we add in advantage from the reckless attacks) - the monk has to go down to 1d8+3 (7.5) damage to get that defence. And the barbarian can keep going for as long as the fight lasts. So, again, not sure what the monk's niche is. Run in and attack for 1d8+3 damage and then use bonus action plus a di to run back out? Why not just have, say, a ranger sit at range and do far more damage for a lot less bother?
At levels 5-10...okay, new monk can run in and get one stun attempt on that wizard and then step of the wind back out. Versus the ranger, who can just stay at range and do way more pure offence against that wizard, or whatever other abilities/spells they have. Is this really a great niche for the monk?
Edit: noticed that I didn't give the monk and barbarian their level 4 ASI but let's call it a wash.