The relevance of the 49% ownership restriction is that any higher non-EU stake would result in the airline ceasing to qualify as an EU- owned carrier. This is important under most bilateral agreements, where an airline has to be owned by either of the two parties to the bilateral. But if DL were to buy out Branson then the airline would still qualify under the EU/US bilateral, as a US carrier.
As for Branson's intentions, who knows. But it's simply wrong to say that he has never indicated any intention to sell. Back in 2010 he appointed Deutsche Bank to assess all options for the future of VS, including the possible sale of his 51% stake. His actions in selling out of other Virgin ventures (endless list...) shows that money talks. If DL were to offer an attractive price I'm sure he'd swallow his pride - as he has had to with Little Red.