As @iacas said, anyone can be a pro or stay a pro, it's not like you get drafted or something.
I think if you play more than three seasons on the PGA Tour you should probably be ineligible from getting it back. Anything less than that there should be some sliding scale, I don't know what that might be but probably in the 5-year range for getting your am status back. If you played mini tours and hardly made any money, then it should be more like a year.
A little pet peeve of mine is when you have some of these former pros who weren't successful as a pro play in am events, do well and people kinda scoff at them, "oh, he's basically a pro". Well no, he's got a regular job now, he was good enough to give it a shot (like many college kids do) and it didn't work out. I don't feel like that player has an unfair advantage or trying to take advantage of the situation. They still have to work at their game and still have to go out and shoot the score.
To me the whole pro thing is mostly just a title, there are plenty of amateurs who are better than pros that play for a living. I'll take Stewart Hagestad over plenty of pros.
There almost needs to be three categories. Tour players, golf professionals who want to play in events against other pros for money and golf professionals who aren't interested in competing at a high level. I guess the line could be where your main source of income comes from.