Australia sure had a strong tradition between 1964 and 1993, didn't they? Those mighty spinners Rex Sellars, David Sincock, John Gleeson, Terry Jenner, Kerry O'Keeffe, Ken Eastwood, John Watkins, Ray Bright, Bruce Yardley, Peter Sleep, Tom Hogan, Bob Holland, Murray Bennett, Peter Taylor, Tim May and Trevor Hohns?
And aside from Abdul Qadir and Iqbal Qasim followed by Mushtaq Ahmed and Saqlain Mushtaq, I struggle to think of many fine upstanding Pakistani spinners either.
I can't see that the quality has vanished, it makes no sense. Simple truth is that England was until 1970 a good place to bowl fingerspin as there was often a decent chance of a rain-affected pitch (and as such we didn't really need wristspinners, and never had any of great quality, Douglas Wright being the best). Since covering that has ceased to be the case (there are 2 or 3 at best, none Test, grounds which offer something to fingerspin in Britain), so even the best English fingerspinners (Underwood post-1972, Edmunds, Emburey, Tufnell, Croft, Giles) have rarely achieved much.