I’m with nok610 on this one. I’m not convinced a graduated scheme would do a whole lot but I honestly believe a harder test would help a lot. Raising the bar on the skill levels needed to be legal has to help. And I would include cars in this. Raising the minimum standard ability of all road users would make us all a little bit safer.
If there is a large percentage of folks out there without a license today then that is a separate problem. Using that as a reason against tougher testing is arguing in circles IMO. If this is as big a problem as you make out (and I have no idea, I always assumed the folks I rode with had a license) then there are ways it could be tackled. Tougher fines, possible jail sentences, better ways of catching offenders. I know in GA I could not get tags for my car without proof of insurance and a valid drivers license. Seems like a simple but effective way to me.