United can be forgiven for not responding to the RA immediately for several reasons. They were on short final, inside the bridge and about 3 out. Landing is not when you expect to do a RA maneuver. They heard Cathay acknowledged the speed and maintain visual clearance. They assumed he was complying. They asked tower for advisories on the traffic, they got them.
I disagree with absolving UAL here. Perhaps their decision can be
understood in the circumstances you describe but their actions were still incorrect. Assuming that other aircraft are follinwg instrucitons correctly, and that ATC have the correct picture, is exactly WHY TCAS was developed...because historically those assumptions have at times proven fatally incorrect.
Doesn't matter where you are on final, doesn't matter if you think you know where the other traffic is or what they are doing -
you must comply with a TCAS RA.
The only missing piece I can see here is the the controller didn't instruct a breakout turn for UAL ( I assume SFO were conducting PRM or SOIA?). Even if they had, however, TCAS compliance in the vertical is still expected.