asked for my Head canon for POI
I don't think this is what you were looking for, but it was the first thing that came to mind. Um, sorry?
I think Finch was right in “The Pilot”- "Sooner or later, both of us will probably wind up dead. Actually dead this time."
I think this show is destined to end with the passing of the baton to another group of individuals in need of redemption and atonement. Finch and Reese are never going to stop helping numbers until both of them are physically incapable. Sure there will be periods of stability and normal day-to-day enjoyment of life, of basking in the newest number saved, but the numbers will never end and each man's guilt from past misdeeds will never cease fueling their need and obligation to try to help others.
It is unsustainable. Finch and Reese are blood and bone, The Machine code and numbers. We've already seen enemies become allies, people we never dreamed helping Team Machine for either their own agenda or for personal demons. And I think this will continue to be the trend over the seasons, lives touched and thus motivated in helping someway. The members of Team Machine losing and gaining members, evolving so that one day, when John finally catches too many bullets or Finch is just a little too late --there will be another admin waiting in the wings and The Machine will find that person another helper or two or three.
And that is how the show will end. With someone other than Finch picking up the phone, the camera panning out, the dates fast-forwarding year by year until one day --The Machine crashes or is unplugged and we hear its final beep. Then the screen will fade to black.