Um, UA flies big jets into SEA. The D gates aren't all that awesome for 737s, let alone 757s, 767s and 777s. US also flies 757s. I don't know if CO flies 757s regularly to SEA, but they are part of the fleet.

I would suspect more likely D candidates will be ones who fly the same gauge planes AS does, so maybe WN/VX or other airlines flying the smaller birds...