I've seen somebody go from being a regular 15 h'cap (over more than 30 years) player to a good 3 h'cap in 2 years so I believe it can be done. The effort involved and the amount of drive was huge, he had a life changing event that gave him incredible drive which I won't go into.
The amount of time invested in practice and money invested in coaching was pretty immense, he went from playing once a week to practicing every day for over an hour, lessons twice a week and playing around 2-3 rounds/week. Still the biggest thing with will/drive, its turning a short term obsession into a daily habit and having total dedication to the task even after disaster rounds. He would be scratch now I'm sure but he didn't have that long to go unfortunately and coming down from 3 to scratch is not at all an overnight thing.
What I would say is there's really more than one type of 15 handicapper, there's those that are already playing the game near their natural ability cap and getting very steady bogeys around the course and there's those that are more wild but have good natural ability and ball striking. He was the latter, and I think this is important, his best shots would make a pro happy even when he was playing off 15 therefore he had a good platform to build on.
Are you sure you want to be scratch in 10 years?
My focus right now is breaking 80 regularly, I know I can do it there's no doubt in my mind that it will take some discipline, my circumstances have changed I can put more effort into the game than I ever could before but mostly I just want it more!