I fly JFK to LAX usually 3-4 times per month on Delta, usually book a week or two out. Of course depends on the day/flight time, but I am almost always seeing between $2,500 and $3,500 (roughly) for one way, of course with some exceptions. Delta One is always full, and in my personal experience, I rarely see more than one or two upgrades clear. Even the 767-300 beats a United 757 imo, which operates the majority of their EWR to LAX flights. United's ground experience at both airports isn't even comparable to Delta's, and JetBlue's JFK lounge doesn't let transcon Mint passengers in (i think you can purchase a day pass separately if there's space).

Long story short, I think Delta thinks they have the best experience on the route, and when it's a Boeing 767-400 or Airbus A330-900neo it truly is the best by lightyears. I think these prices are here to stay, and I think people are paying them.