Haha, it's so ludicrous for anyone to say a originating from a certain country automatically means a batsman is good against spin.
There are many Australian batsmen who are good at playing spin, and many who aren't. Same with India, Sri Lanka and the rest.
To suggest that the likes of Hauritz and Cullen are ever going to make Test-standard bowlers has always been laughable to me, I don't think many people imagine the likes of Casson are ever going to get too far, and it's pretty convenient to suggest the Baileys of this World have poor records just because they get played very well. The whole point of a good bowler, and a wristspinner more than anything, is that you can play them as well as you want, they'll still get good figures.
Right now, the cupboard of spin in Australia is bare. Whether someone like Cullen Bailey steps up in the next few seasons waits to be seen. I think it unlikely myself, given the fact that Grimmett\O'Reilly\Warnes have historically not come up without gaps in between.
I think that is silly logic really - assuming that someone won't be good based purely on the fact that another good player has just retired and there have historically been gaps in the process.
The chance of a good spinning emerging now is the same chance it would be at any time. You spoke yourself about it being ludicrous to suggest that a country's spin stocks will continue to follow time patterns over history. As you said, "The whole point of a good bowler, and a wristspinner more than anything, is that you can play them as well as you want, they'll still get good figures." - well, it doesn't matter that Warne has just retired, or that there are typically gaps in the process, if Bailey is a good bowler, he will get good figures. The likelihood of him being a good bowler isn't any less just because Warne just retired, nor is the likelihood of a good bowler coming through.
Now, I highly doubt you've seen Bailey bowl, so you've instead decided to judge him based on his figures and some historical patterns. No-one could possibly say he is test standard right now, but no-one, without seeing him bowl, should be able to make sweeping claims about Australian spinners based on the history of how often they come along or the likes. It'd be like going to the Rashid thread and saying it's highly unlikely that he'll be any good purely based on the fact that he's English and England haven't produced any good wrist spinners at all. The fact that his English really has no bearing on how good a bowler he is (*cues BLE*).
I've seen Bailey bowl a few times - he looks pretty good to me in his action and his approach. Obviously he is not test standard at this stage, however he doesn't have anything intrinsically wrong with him or his action to suggest he can't develop into a test standard bowler, and unlike in England when things so often go incredibly wrong with potentially good bowlers who eventuate into rubbish, the Australian system generally works in a way to get the most of a bowler like Bailey providing he has the right attitude and doesn't have any technical faults. He struggles with consistency game-to-game which is one thing that generally always improves with young bowlers (in Australia anyway) and he certainly needs to develop a quicker, flatter delivery or work a bit more sideways drift in the air into his bowling to stop batsman coming down the track to him with such confidence. To say he is test standard now would be ludicrous, but to make a judgement on a young bowler in development, whom no-one is claiming to be test standard now, without even watching him bowl is equally ludicrous, IMO.