One thing you guys aren't factoring in is loyalty.
A lot of you are just outright assuming that people will return for any future Diablo games when there is an unquestionably better option on the market for free in PoE.
A lot of players, myself included, were burned by Diablo 3. It had a disastrous launch, and at times an equally disastrous development cycle, before development was totally shut down.
Even if its a new class, it won't be that big of a deal. Necromancer wasn't the smashing success it needed to be to revive the game. If they wanted to do that, they would've had to implement a hardcap on paragon, bring real trading back, and massively reduce the effectiveness of ZDPS cheese builds, while also producing a steady stream of content.
They will not do that, and thus the game (and franchise) is dead until further notice. Should the day ever come where a D4 is on the way, people will remember what Blizzard did with the potential of D3, and a lot of them will pass.