I love the ending of regular show, it was build on the two drive force of the show: maturing and weird shit.
Pops were there from the very beginning (the shorts) as the weird creative face of the creator when Mordecai and Rigby were there to be a reflection on growing-up to adulthood.
And the ending was the closure of all that with M&R growing to a good fulfilling life, separate but still friend.
Pops as a "god" observing his friends concluding that RS was good and move on (we have blurays to watch it again).
It was bittersweet (as life is) and even if all wasn't plan from the start exactly, the team of creators new where they wanted to go, I think, specially on M&R growing up.
With time and popularity they found the specific way to a really good send off.