This benchmark was certainly considered in the past but is now obsolete. The 5-man rotation has been in place for most teams for nearly 50 years. Starters are pulled earlier than ever before because most baseball people believe that using a fresh reliever is more likely to result in a win than using a starter who has thrown 80-100 pitches. For these and other reasons, pitching wins has become an increasingly unimportant statistic, especially regarding relievers. If you want to consider a simple counting stat, innings pitched is probably a better indicator of overall effectiveness, especially for a starter, and even that is overly simplistic.
The active leader in wins is Justin Verlander with 226. The only other pitcher over 200 is Zack Greinke with 219. (J.A. Happ, nobody's HOF-er, is 9th on the list.) I don't think it's a stretch to predict that no pitcher will ever again register 300 wins.