I am by far an expert on anything UA; however, an empty row never seems to stay an empty row on any major airline, especially hub-to-hub. The best bet is to choose the routes that are less popular. Ie, I fly AA's DFW-MCI often, and on Saturday upgrades are not unusual for me, nor are empty rows. On a weekday? Forget it. Business travel is a thing. All the time. It's really surprising to see the contrast, actually. Most routes have less traveled times or days. But there are still no guarantees that middle school field trip didn't select that flight from LAX to DEN because it was the best deal...