Aircraft shouldn't be a problem, ORD-PVG is only 7,057mi. Even bypassing Russia would be around 7,500mi +/-. That's within range of any 789 even with strong headwinds. But then flight time and carrying additional fuel would make it close to ULH territory and I'm not sure UA wants to deploy its aircraft like that. Given that they can perfectly funnel traffic through SFO at a lower cost.

DL is kinda stuck with DTW since SEA is apparently not producing the kind of yields they want. And ATL would be too far.