If seat blocking means, UA can not sell the seat or sell E+ access, than forget about it.

If seat blocking means middles are filled based on status (a seat between 2 1Ks would be the last used), then perhaps.
BUT one needs to realize this creates a siginificant complexity in assigning seats especially for parties that need to be split up and would atleast like to be near each other in the cabin.

I think in the age of high load levels (even if some flights are not full) and the ability to upsell to E+ this a complexity that is not likely to return.