Well they may surpass UA, but they can still only dream to be as truely global in KEY world financial and business centers. DL does not offer 3-Classes of service on their international widebodys (metric of being a premium airline, IMO), they don't have anywhere near the amount of 777s that UA has and has had for the last 10 years (52), no 747s (UA has 25), and they fly to a lot of secondary cities around the globe. United has large operations in London Heathrow, Frankfurt, and Paris (the places that actually have most revenue), and has tier one Asian destinations (and slots) in a stranglehold (Hong Kong, Beijing, Tokyo Narita, and Taipei). Not to mention Sydney, and Kuwait. Their domestic hubs are arguably the most sought after soley for their geographic location. So yes, perhaps DLs traffic will surpass UAs, but it won't be the same high yielding traffic that UA procures with their top tier international destinations around the globe.

Now it's up for debate how well UA is leveraging their painfully evident competitive advantages, but the substance and revenue of their route network is not. It doesn't matter how many new African destinations DL expands into becuase on planet earth, those can't compensate for UAs service to key world centers.