Ultimately, once you look past the hubris that both DL and UA display these days, it’s ultimately about pricing power out of their hubs. I.E. UA can have extortionate pricing out of SFO and similarly for DL out of JFK. UA in J on SFO-EWR is equivalent to DL on JFK-LAX these days (2,500-3,000 one way before corporate discounts).
I think they’re both doing pretty well where they are strong.
I agree. United has the benefit of being in a better alliance, but Delta's DOne and lounges are better that UA's Polaris and United Clubs.