Actually it's quite possible it may be better to use the A350. Supposedly the premium load factors for Delta are high, while the economy is almost empty for Delta. The A350-900, especially the 40 seat config one, has almost a dozen more Delta One and Premium Select seats and 30 less economy seats compared to the A330neo they currently use there. A Delta One passenger is worth at least 3 to 5x of an economy passenger tbh, with only marginally higher costs, it's a pretty good tradeoff if they can fill the front of the cabin.

The only issue is I don't think Delta has an A350 pilot base at SEA.