For me, only confirmed upgrades (PlusPoints) at booking, possibly consideration of R fare (lowest PE fare), or scorching cheap paid upgrades are in the cards. If I can't be sure of an upgrade/likely upgrade, I am team lowest fare, and spending the $ elsewhere - e.g. at destination.

To its credit, United has basically perfected the art of premium cabin monetization. I am not playing lottery odds, or refreshing upgrade page every five seconds.

Full disclosure, I may be one of the few on here who book and fly on basic economy fares (having logged over 100K miles on BE), and/or fly carriers like F9 (Frontier), so my POV may be different than most frequent flyers.