I see a lot of discussion about how good the Delta onward connections will be for Virgin pax going onward in the USA.
However, the current Virgin routes, and the Delta network in the USA, do not mesh together well at all. The main hubs for Delta are (in rough order) Atlanta, Detroit, Minneapolis, and Salt Lake. Virgin serves none of these.
Virgin's principal operations are to JFK, Los Angeles, etc. Although Delta are a major player at JFK, they do not have a lot of domestic connectivity there, most are low frequency and/or to other major places already served directly from London by one or the other of the two carriers. If you have ever actually tried to make a connection at JFK, it's probably the worst gateway for that in the USA. Delta's main domestic hub in the New York area is now La Guardia, which they have built up substantially in recent years.
Los Angeles is in a far corner of the USA and offers few onward connecting possibilities.
I certainly don't see Delta handing any market share from London to Atlanta or Detroit etc over to Virgin; these are some of their most profitable routes.