After Johnson, 46 year old Jamie Moyer and his 249 career wins are next. Roy Halladay, a possible candidate has only 139 wins and is 32. According to a recent Sports Illustarated article, he'd need 16 wins a year for the next 10 years to make it. Youngster Tim Lincecum is 25 and has 28 career wins. That means he needs 15 wins a year until he's 43. Impossible? Well, no. Unlikely? A definitive yes.
In the era of keeping pitchers in a crib for most of their early years (see: The Joba Rules), teams are very protective of pitchers and any scraped knee will probably lead to a trip to the DL. It is very difficult to be healthy for long enough to make it to 300 wins. You also need to pitch for a pretty good team for most of your career. Because of these factors, Johnson will be the last 300 game winner (until someone else proves me wrong).