I don't know what you mean by "main feature" these features were not effective at high level. they were weak unless you had a subclass boost to FOB. Other uses of ki are more effective, way more effective.
What is not weak is the Monks other abilities and some of their subclass abilities
They do suck but other abilities don't, particularly when combined with Monk or subclass features.
Proficiency in all save is WAY better than these and ALL Monks get that
Resistance to ALL damage is WAY better than these and ALL Monks get that
Being able to end Charmed or Frightened automatically with an action is better than these at high level
The basic free Monk movement is better than these at high level and step of the Wind is generally a better use of ki.
Evasion is better than these at high levels, especially when combined with resistance.
That is before we start talking about subclass abilities and all these things are better than Flurry of Blows or Stunning Strike at high level.
You still have not answeredmy question though: How many Monks have you seen played at level 20?