Circa 2019, when UA had 8+ flights per day every day on SFO-EWR and LAX-EWR _each_ and 4 SFO-BOS as well as 5+ SFO-IAD _and_ 5+ LAX-IAD, 21 day advance Y- purchases in the $300+ range were quite common.
Now, UA's flying half the number of flights.
The seats and the same prices aren't there right now.
David