Where do you get the idea DL has loyal following in the Philippines?
DL has a very loyal following in the Philippines. I know because I'm part of that loyal following. Same with my dad and grandmother, and that's a following that dates back to the NW days. There are Filipinos, particularly those on the East Coast, who will only fly DL to MNL.
DL doing ICN-MNL is for the benefit of the joint-venture with KE and allows them to circumvent S. Korea - Philippines bilaterals by putting passengers on US-registered metal on a US-based airline. If KE were to get more frequencies for ICN-MNL, the DL 5th freedom flight is like going to get axed.
You know DL's flights use Korean entitlements, not American ones, right? DL had to cannibalize KE's entitlements in order to make room for it to fly to MNL. So no, no circumvention took place, despite what you're claiming here.
Furthermore, yields on US mainland to SE Asia bar SIN are trash courtesy of Mainland Chinese airlines. There won't be a UA SFO-MNL or an AA LAX-MNL or a DL SEA-MNL.
Most traffic going from the US to Manila will either fly PR or do a one-stop for either cheaper fares, wanting to avoid PR, and/or wanting better premium product that can be provided with JL/NH/CX/BR/etc.
I wouldn't discount the possibility of a US3 airline launching direct service at some point. There seems to be this notion that the Philippine market is notoriously low-yielding, but I'd argue that it's not as low-yielding as people make it out to be here on A.net. There is
economic growth happening in the Philippines, and if there is sufficient market demand for PR to support 17x weekly ervice on MNL-LAX during wintertime, what's preventing any of the US3 from launching that service given the demand for it?
While yes, I primarily fly MU for cost reasons, I wouldn't hesitate to shift to DL if they were to fly LAX-MNL (or SEA-MNL) and I have a reason to fly them. I imagine many other Filipinos (and Fil-Ams, and other Americans) would do the same.