Sure, I don't disagree with this. I think we use 'luck' as a high level term to describe non-sustainable events or pieces of events that have been largely proven to be non-sustainable. Wins are wins, however you got them, they are banked and credited to your team, and the Cubs haven't gotten enough of them one way or another. The simplest way I can describe it is if you declared tomorrow that the MLB season would go another 100 games starting Monday, you could make a strong argument that relying on pythag over W/L records YTD would be a better indicator of what your record would be for those 100 games. Obviously we're coming to a significant cut off point and rosters will churn and guys will grow 6 months older and pick up and lose skills and injuries and I think we're all aware of that.
I think CubinNY would disagree with the statement that the job of an offensive player is to produce runs. He would say it's to win games, I think. Apologies if not. And, honestly, it's not an incorrect argument. But all of our individual statistic tracking (that drives 'player value') is based on a players ability to produce runs (or avoid outs). And vice versa for pitching and defense. And so a statistic that looks, at a macro level, of how many runs an offense were produced vs how many runs were allowed, with no regard to how those lined up in 9 inning chunks, is beneficial in predicting future performance.