Haven't flown United lately but they did some boarding trials in 2015 that I observed at ORD.
At that time at least 1Ks didn't preboard and were BG1 with plats, I know things have changed. Boarding order then in the 2015 trials was:
Originally Posted by phltraveler
Preboard disabilities GS uniformed military (straight line into gate)
1K/First class
Premier platinum
Premier gold and star gold
Group 1 (premier access)
Group 2 (no notation)
(subsequent groups not observed, had to board another flight)
Effectively at that time (almost ten years ago) it was a demotion to Premier Plat as 1K was split off (apparently irrelevant with 1Ks preboarding now anyways), and premier silver/purchased priority boarding wre demoted as they were split off from BG2 with the golds.
Again, this was ~7.5 years ago and things change and United may be trialing something totally different here, but just perspective on a new boarding process they trialed and ultimately shelved. It was more groups, not fewer, but they tried to funnel it with alternating lines of "boarding now"/"boarding soon" (with "board anytime for previously called groups in front of the gate) to avoid the gigantic BG1-BG5 lines spilling everywhere.