Eh... I'm on the fence about Ashton Kutcher. I'm surprised Chuck Lorre and CBS jumped for him, as opposed to hitting for Hugh Grant quality. But since he's sitcom-experienced and a consummate professional, I guess that's all that matters. But Matt, after all that s**t Sheen put Lorre through -- he deserved to be fired. No one's irreplaceable, especially after biting the hand that feeds you for years.
The key between the revived show going for one final season as opposed to an additional three or four is the chemistry. If Kutcher hits it off chemistry-wise with Jon Cryer and the supporting cast -- it will be a hit again. The writing is a key factor too, but Kutcher has to click with the cast for the show to enjoy another revival.
And if Kutcher gets Natalie Portman, or Mila Kunis to do special guest appearances.... more power to him. And more Melanie Lynskey while we're at it.