The only pilots that have made a fuss about the safety of flying to HK have been from the airline named American. I'm not aware of any similar action at United.

Cancelling flights to HKG for AA and UA make perfect business sense (given dramatically lowered demand, on top of the protest situation, and the win-win of offloading flying to CX for AA). Singapore doesn't have the same business considerations. Sure, it's possible demand is down, but certainly not as much as HKG, and given the harsh competition between SQ and UA on the SFO route, I doubt UA would be willing to cede it entirely to its main competitor. Perhaps they'll reduce frequency, but zero chance they're happy cutting it entirely.