This is all very interesting - certainly I agree with you about Warne, I think he could have got many more wickets than he has.
In a sense, the "conventional" wristspinner has never been a good bowler - he has been someone like MacGill, who will turn-in the odd match-winning performance but by-and-large will be too expensive (and cost too many games) to be really considered a Test-class bowler.
There have only ever been a few genuine Test-class wristspinners that I've heard of (obviously, your scope is far larger than mine and you might like to add to my list), those being Barnes, Grimmett, O'Reilly, Benaud, Abdul, Warne and Muralitharan (Kumble for most of his career spun with as much his fingers as his wrists). Mushtaq's career, sadly, declined after 1997, otherwise he'd have been part of the list, too. Danish Kaneria could well be added in a few years' time.
All were special because they could control wristspin the way most are unable to.
My original post, though, you might notice, related to [bi]fingerspinners[/b]. In the age of uncovered wickets, pitches that helped fingerspin were commonplace anywhere in The World you wanted to go, but most especially in England, where it rained (and still does) more than anywhere else.
Nowadays, the only places where fingerspin-friendly wickets occur regularly enough to help a bowler be of use to a team are in the subcontinent. So it's no real surprise that most success for fingerspinners nowadays occurs in the subcontinent. And that the bowlers who play regularly there (ie are Indian or Sri Lankan especially) are those who are most worth their places, and have the best averages.
However, someone playing most of their cricket in the rest of The World will invariably struggle to bowl fingerspin effectively, because most places don't produce sufficient turning pitches to make them useful.