You seem to have a strange idea that Diablo 3 should be a game that you can -- and indeed, MUST, if you want to get anything approaching an ideal set of items -- literally play forever. Why should Blizzard care if the carrot can be reached in a sane amount of time? It's not like they're collecting a monthly sub from anyone.
People are giving up once they get to the point where it's obvious that no further upgrades are attainable without exponentially increasing the amount of time played. How many do you suppose gave up after seeing what it's like on one character and realizing that it would be a complete waste of time to do the same thing again on another only to hit the same wall? If, instead, the game let you get what you want in that same amount of time, how many would move on to another character and effectively increase their time played by up to 6x?
How is any of this related to the "It's not like they're collecting a monthly sub from anyone."
My monk was ranked 240 on d3progress (NA) for elemental elite dps 2 weeks ago, I'm now down to 628. For a few people, keeping up with the joneses is pretty important, and would continue to play that character to maintain it's fairly high ranking but getting that 1% upgrade.
Me? I started playing my Witch Doctor (after rolling a new one that was female because the meth shakes of the male one annoyed me), which is now ranked about 15,771 in NA. My gear is sub optimal, it's a work in progress, and that's important to me. I'm actively playing a gimpy version of a WD to get better shit so I can stomp faces with an awesome WD.
Increasing my play time with a certain character grants me better gear over the long term. I won't be smashing T6 any time soon, but that's the goal. Blizzard doesn't profit, or lose profit by me playing or not. 3rd party sites don't profit from it either.
Turn trading back on, and all that changes very, very quickly.
The only middle ground I've thought of is a second server set that allows trading. Kinda like FV was for EQ (no xp bonus though). That way those kinda people can play in their little neck of the woods, and I can play in mine. Softcore, Hardcore and Lootcore. Yeah, that sounds awesome.