I don't buy the "as CD's are to records, so transistors will be to vinyl" argument for one simple reason-
A CD is supposed to be a flawless reproduction of an acoustical event.
A signal coming from a guitar amp, however, is not.
Digital is undeniably a cleaner medium than analog, but if cleanliness were what we were looking for, we'd all be plugging into the board direct, with nothing in between. We're looking to color the signal, and due to their inefficient nature, tubes do that quite nicely.
I think amp modeling has a tremendous amount of potential, and once companies like Line6 get off their "let's make this sound exactly like THAT" hangup, will push the boundaries of what was considered possible in a guitar tone QUITE a bit. However, at the same time, a tube amp has a very different response and feel than it's transistor bretheren, and i doubt it'll be superceeded anytime soon.
It's kinda like asking, "digital cameras offer perfect reproduction of anything you point them at. So, why do people still spend thousands if not millions on those old-fashioned, blurry, sloppy paintings by guys with French names? I mean, Monet? who the hell did HE think he was?"