Truth of the matter is, spin bowling has never been as impressive an art as seam-bowling, not since covered pitches. They said it had "been killed" in the late-1970s and most of the 1980s too. Truth is, bowlers like Warne are generally once-in-several-generations bowlers, Murali is perhaps a one-of-a-kind bowler who we'll never see the like of again; and as Halz pointed-out, fine and long-lasting bowler though Kumble has been, he's not been any greater than someone like post-covered-wickets Derek Underwood, Ashley Mallett or Iqbal Qasim, though he's played way, way more cricket. I see MSP's career taking a similar pattern, though obviously he's a rather different type of bowler to the four aforementioned.
Harbhajan Singh has been a bit disappointing of late, but I still think he has it in him to be a good spinner on a proper turning surface. The question is, how common is the turning surface going to be? We're seeing less and less of them in India of late, and though we have seen far more than you'd have expected 8 years ago in England in the last 4 years, I hope that doesn't last.
Kaneria's career looks like it's going to fizzle-out, has been a long time since he's done any serious damage to Test-class sides and even then nowhere near as much as his predecessor Mushtaq Ahmed used to do.
What I hope more than anything is that Sri Lanka end-up having a Kumble-like bowler whenever Murali finishes his career. If that doesn't happen, I see them struggling badly even at home, something I very much don't want to see.
Seriously, though, spin has been a secondary art ever since covered wickets. It's only going to be when the likes of a Warne (and it was remarkable that his career coincided with Muralitharan's) are around that it's going to come to light again. And as I said, I don't see another Warne any time soon, and nor I doubt do many other people.