Clearly, you do not know how most folks are booking award travel.

There are lots of last minute availability for saver seats on routes that have excess capacity. Always has been the case. But that is useless for most people's redemption needs. Most people are using award travel for vacations, which requires some level of advanced planning. Last minute availability does nothing for them.

That said, I do agree with your general view that UA is better for award availability that most others, particularly CO.