The Way of Mercy Monk is a pretty cool class in my opinion, I like the healing, as well as the ability to do extra necrotic damage. But what I don't get is the capstone feature. Mechanically speaking, what use would there be in putting someone in suspended animation for 17 days? I can't imagine any reason for a PC to need to be put out for that amount of time.

Well you can do it to unwilling creatures too....

So you could capture the BBEG potentially and bring them to justice. 

True, but any BBEG in a campaign where you're hitting 17th level (and potentially higher) likely has a Con Save high enough to easily pass, or at least legendary resistances.

I kinda agree this capstone feature needs a second part (since at 17 most will be immune to incapacitation and poison, making earlier features less valuable).

I'd suggest: "Urgent Unguent" or similar where: Monk may move up to double their speed (and double jumping distance and height) as a reaction when moving to an creature that has just been incapacitated. Once this feature is used it may not be used again until the monk completes a short or long rest.

This way if facing Tiamat or other that can't be affected by the Noxious Aura or Hands of Harm, the monk can at least get to the primary healer quickly if they go unconscious.