AA offers Flagship Lounge access on transcons, but UA does not. My suspicion is that DL will let transcon D1 into the new lounges but who knows.
This would be part of being competitive on the LAX-JFK and vv route. If certain other hubs like SEA and ATL don't have a D1 lounge, it wouldn't matter as much for some routes, although if there's a D1 lounge at JFK, it would be hard to permit only some D1 routes touching JFK to use that lounge.