Stick with UA given you are based out of Denver, plus UA (and possibly NW) pretty much dominate transpac routes. Plus star alliance partners (AC,OZ, etc.) gives you more flexibility on transpac. Plus all the SWU UA gives you, I would stay with UA.

But Europe you have to rely on Lufthansa.

Have you considered joining something like NW as a second option? NW have a fairly good transpac routes, but they are pretty good for Europe also (with KLM and Skyteam network)