I've switched to AS and DL and I'm very happy. With AS I can bank my occasional AA travel and get decent seating with my MVPG status. This year while testing the waters I also banked all my DL travel there but soon starting a plat challenge with them. Delta has plenty of options with flights to SLC, ATL, and MSP and great connections from all 3. They also have higher frequency to LAX than UA, and you can use AS to connect to the new network developing in SEA.
So, this AS and DL thing is working for me so far. Next year I'll be heavier invested in DL than AS, but expect to maintain status in both.
US has good lift but both eastbound transcons are seasonal and redeye which is awful.