I completely agree that seat blocking (subject to availability) will be a differentiating elite perk. Huge difference for those of us who fly ULH/LH. The implementation mechanism could be relatively easy with no variable cost to UA. If not reverse upgrade list order per fumje's suggestion, the algorithm could simply allocate standbys and BE in a way that ensures blocked/empty seats next to higher tier elites are last to be filled.Some GAs already do this and others do so upon request. One particularly positive experience I had with UA was when the last empty seat onboard UA28 was between me (1K) and my travel companion. No GSes in cabin so the GA proactively moved the last standby pax into the one other available seat.

But it's currently inconsistent so not a perk per se.