Except that Richie Benaud, the person who has attended more live games of cricket than anyone in history (as a spectator, player and commentator), who saw Grimmett and O'Reilly growing up and himself being a leg spinner also claimed that Warne was the greatest leg spinner in history. He was willing to claim this from very early on in Warne's career. Forumers tend to rate players based on their longevity far more than former players.
Finger spin is far easier to bowl to a competent level than leg spin due to human physiology. It's a lot easier to control the ball and get enough speed on the ball to be threatening as a finger spinner. There's a reason you more often see part time finger spinners than leg spinners. And finger spinners can tie down an end even when they're not threatening the batsmen far more easily than leg spinners.
There are a ton more leg spinners in world cricket now than when Warne started. When he started the only leg spinner of note in world cricket for the previous thirty years was Qadir. Since Warne Australia has played - MacGill, Hogg (not technically leg spin but the left arm variety), Casson, McGain, Smith, White, Zampa and probably a couple more who I'm forgetting. Lyon is our spinner now and we've tried Beer, Hauritz and a few others.
Warne showed everyone what a leg spinner could do and world cricket sat up and took notice. Did you ever wonder why there are so many wrist spinners in world cricket coming through right now? They're the kids who saw Warne, said "that looks awesome" and tried to start bowling leggies.
An argument from authority using a single source is extremely weak (especially choosing a source from his country), but even if he was correct, so what? Ive already said I don't mind people claiming reasonably early in his career that he was the greatest spinner ever (this is actually more than Benaud is saying because he refers only to leg spinners). But this isn't a reason to reinvent the history of cricket to give him some ethereal impact on the game to try to make him look even better.
And of course longevity matters in a discussion of this sort. See Steve Smith.
In a discussion of the ATGs, are you really trying to construct an argument about finger spin being easier because it's easier to be mediocre in it? How is this relevant? And this despite leggies outperforming offies through most of the course of history.
There were 3 good (or at least decent) wrist spinners actually playing when Warne debutted- Qadir, Ahmed and Kumble. The last time there was an offspinner that was of this quality was the mid 70s probably. You don't hear people claiming Murali revived off-spin.
Are there more leggie playing now than when warne started? Yes, but this is a combination of limited overs (in which the dominance of leg spin came long after Warne, only in the last few years), more matches being played, and more players being fielded. There are a lot more seamers too, and a lot more off-spinners. And unlike pre-Warne, we actually have several teams fielding good off-spinners, while the number of good legspinners is probably less now than when warne debuted.
Australia have tried a number of leggies since Warne. Is this Warnes impact or the fact that AUS have produced a large number of top leggies and not a single top off-spinner? Who knows, but I dont think Warne would want to claim a legacy of creating a revolving door policy on mediocre leggies that weakened the side for years until they eventually settled on a good offspinner.
Kids emulate all top players. Warne is not unique. The kids that were watching Warne play in the 90s are approaching cricketing retirement age now. This argument is a bit ridiculous. The only country that was producing good leggies in the 2000s was Pakistan, who were producing good leggies themselves before and during Warne.