Creating the Diablo 3 public server will be a beast amount of work. It's not like enemy units are as simple as they are in MMO's - movement patterns for one will consume hundreds of hours of the developer's time.
Personally, I would rather make my own Diablo clone and sell it for riches and lord it over them that I finally made a game strictly better than Diablo II than to take up the yolk of furnishing a D3 server. You'd have competition, and the rewards would be nascent.
That said, fans spend hundreds of hours building emulators and translation patches of old games (or even in the case of Quest for Glory 2 - AN ENTIRE FREAKING REMAKE)... there will be a decent (but perhaps not great) Diablo 3 public server one day. A day much sooner than 12 years from now, at the very least.
I find that comforting in a strange way.