It is perhaps an unpopular opinion, but I have always thought that if Boeing pulls out of Everett, that's now your solution for land to build a relief airport for SEA. Personally I think that's more a matter of when, not if. Mukilteo's city council would throw a fit and sue to delay the project, but that is not a showstopper.

The only place I typically fly from PAE is LAS. It is point to point, and if cancelled, I can make a change and get a flight out of SEA easily enough. When I fly to LAX, it's almost always to connect internationally and I want the frequency that SEA offers in that respect to cover for unexpected delays or cancellations. That's why I don't think ORD or DFW (with onward connections to AA) would work - too many cheap flights to those destinations from SEA that leave all throughout the day. If an Envoy flight is late getting in from a more rural airport, which happens all the time at those two airports... you might be waiting a day to get back to PAE or you fly back to SEA.

On the other hand for ULCCs, which typically operate odd schedules from secondary airports (ATW-PDX on G4, anyone?), I think it can work pretty well. F9 to DEN opens up connections eastward and I will hazard a guess it's cheaper than whatever UA was offering years ago from PAE. I think they will do OK with the cost conscious crowd in the north sound.