Wins definitely aren't the end all be all of pitching statistics, but to say they're completely irrelevant is wrong. The pitcher at least did something right to get the decision. I'm not saying 300 wins in 27 seasons is anything remarkable because it works out to basically 11 wins a season, but it is remarkable to be a dominant pitcher for 27 seasons.
Wins are irrelevant.
I don't think that anyone would argue that they're completely irrelevant, but they're an extremely overrated statistic. Too many factors beyond the pitcher's control factor into whether or not he gets a W.
They're a name that was attached to a certain outcome. To focus in on SP; if a pitcher completes five innings, with the lead, and the team goes on to win without surrendering the lead, the SP is attributed the "win."
What is valuable is the quality innings pitched
1) Pitcher A exits after 6 innings pitched, having surrendered 4 earned runs and his team leading 7-4. The team goes on to win with the SP is credited with the win.
2) Pitcher B exits after 6 innings pitched, having surrendered 2 earned runs and his team trailing 0-2. The team goes on to lose with the SP is credited with the loss.
Pitcher A won, Pitcher B lost, yet clearly, what Pitcher B contributed was more valuable than Pitcher A. This is because it's the innings pitched that are of value, not "the win." That's not to say that what Pitcher A did was not valuable, just less so than Pitcher B.
**I'm dumbing down the situation. With advanced stats, it is possible to conclude that one pitcher is more valuable than another pitcher despite allowing more earned runs as a result of defense, park factors, luck, but the general point remains the same.