Honestly, I would argue DL doesn't give a sh!t about PDX. Sure they probably make a little bonus cash & yes, they have history at PDX too but they would much rather allocate their resources elsewhere. Their PDX network is just enough to fend off/keep competition in check. PDX helps them either lose a little less or make a little extra cash while protecting their nimble (compared to UA/Star & AA/oneworld) west coast international market.
DL knows PDX isn't strong enough for many players and PAX would rather fly PDX-Connect in Asia-Asia Destination rather than PDX-SEA-Asia Destination. Image what kind of dent it would put in their ambitious SEA OPS if an Asian carrier (JL/HU/CZ/maybe CX) teamed up with AS and started PDX - there would be connection opportunities at both ends; plus cargo.
The move to HND is merely to occupy a spot keeping it from someone else who could operate (as example) HND-ONT/LAS/PHX/DEN/MIA which again, would further injure DL's weaker Asia network.
Honestly, I would argue that you have next to no knowledge about a.) the PDX market in general and b.) even less knowledge about airline revenue management and route planning. To paraphrase, you clearly don't give a $hit about facts when posting on a-net on topics like these, based on what you wrote. Did DL lose your bag once or re-route you unexpectedly? Yikes.
DL is the top dog in PDX...they know it. And they *DO* give a s--t because they are clearly growing there, they have noticeably increased their international presence and maintained it, and they have a large, loyal base of customers in the PDX market, some of which came over from NW. DL knows exactly what they are doing, and meeting the demand very, very accurately. DL, and other established, profitable airlines engage in what is known as "game theory"...knowing what their market position is and that of their competitors, and then anticipating what their rivals next logical step would be, and then making a move to stop their rival or potential rivals.
Example? Take the PDX-LHR route. DL saw no other US or UK carrier was entering the market, so they started with seasonal, summer only, a few days a week. That went well, but sitll no entry from a rival on that route. The closest would be Icelandair, with a stop in KEF, but Icelandair still hasn't committed year-round, daily service to KEF with connections to Europe. So then DL upped their game and made PDX-LHR DAILY, but still for the summer season. No clear rival yet...and the chance now there ever will be is declining. Another year or two of market development, and PDX-LHR will be daily year-round, or very close to it. Someone at British Airways was asleep at the switch when it came to PDX...they could have put on a 787 on it, but didn't, and now they are on the outside looking in. The PDX-London market would have to double or triple overnight to support another entrant, and that isn't likely to happen anytime soon.
The same concept applies to HND. DL knows the PDX market, and how it ties in with their operation in SEA. Airlines like JL and NH may have looked at PDX once or twice, but sees who their competitor (DL) is and passes. With the new HND route, the flight will depart 3 hours later, making it the last US-HND flight of the day on DL. That basically makes the PDX-HND a reliever, especially for excess demand in SEA. Watch DL's summer 2020 schedule for PDX...look for re-timing of connections from SLC, LAX, and other cities to time with the mid-afternoon flight from PDX to HND. All other HND nonstops leave at 11:00am Local time.
As for PDX: demand is very sensitive to the performance of the local economy, much more so than larger cities in the region. The peak travel season is primarily March/April through early October. PDX fliers who travel regularly either belong to Delta SkyMiles, United Mileage Plus, or Alaska's Mileage Plan. Those fliers stay loyal to their airline to maintain their status, and the route and frequency offerings are meeting that demand. UA and AS are direct competitors on several routes; yet UA and DL really don't compete with each other at all out of PDX. I cannot understand why F9 or Spirit or JetBlue bother serving PDX at all. AA doesn't seem to care, either, and never have. UA used to be very big in PDX...even had a PDX-NRT in the 80s one day a week, and widebodies to SFO, DEN, and ORD. But in the 90's, they really lost out to WN and AS on the predominant flight pattern out of PDX: other west-coast destinations like SFO and LAX. WN and AS simply beat out UA with lower prices and more flights.