I'll bet you $50 that there's at least 2 pitchers active in the majors this year who eventually get to 300 wins, not counting Johnson (who I'm think might be done). (And we'll probably have to wait about 20 years to find out for sure, but that's OK--I don't mind waiting to collect.)
Mussina still has a chance, at least, though I don't think he'll make it. Some of the other guys who currently have 200+ wins I suppose might make it if they last as long as Ryan did, but the best bets are younger pitchers who are not anywhere near that yet. Johan Santana is 28, and is already up to 89 wins, and C.C. Sabathia is 2 years younger and has 6 more wins. Do you really want to argue that it's not possible for either of those guys to make it to 300 wins? And if neither of them do it, maybe Roy Halladay or Dontrell Willis will, or maybe Ben Sheets. Or maybe some kid who's in his first season, and will end the year 2-3 with an ERA of 6.49 (Santana's marks his first season). While the odds will be strongly against any individual pitcher we might look at making it, there are enough pitchers who have a bit of a chance that the cumulative odds are that someone will do it.