Who knows why Delta chose to launch two routes with little feed on either end to compete against two carriers on each route with a LOT of feed on each end: SEA-LHR and SEA-DFW.
What feed does AS have on the SEA end (relative to a passenger arriving from DFW or LHR) that DL doesn't have? You're making it sound like DL has a miniscule operation at SEA - they can flow a DFW-SEA-Alaska/PNW connection just as AS can and they can also flow DFW-SEA-Asia connections. Similarly from LHR they can flow LHR-SEA-Alaska/PNW/Hawaii/California connections just as AS would.
~Twice as many seats on AS vs DL and I’ll let you count the destination difference. Absolute destinations difference isn’t huge, but it’s still a difference. Seats to those destinations is bigger on Alaska easily.
Just because AS has more feed, it doesn't equate to DL having "little feed" as you suggested. Sure, AS serves more destinations in the PNW that would logically connect to DFW or LHR over SEA, but what's the combined PDEW of WRG/EAT/PUW/HLN, etc. to these destinations? DL serves FAI, ANC, GEG, PDX, BOI, SMF, Hawai'i, etc. from SEA, which is where the lion's share of connecting passengers would likely be originating/terminating.
Additionally, if in the future DL needs to route more LHR/DFW-bound connections over SEA in order to supplement the O&D traffic they're able to pick up, they have more options for accommodating other passengers who have been connecting on these flights over their other hubs like SLC and MSP. For example, say DL wants to free up 10 more daily seats on a particular BOI-SEA flight that connects to the new SEA-LHR service. In order to do that, they may need to route some passengers who have been flying BOI-SEA-BOS, BOI-SEA-SNA, etc. over MSP and SLC, but they have the flexibility to do so.
This is my signature until I think of a better one.