It must be the routes I fly, but i rarely see empty seats (or just a handful) on all single-aisle domestic segments I'm on.

I'm fine with the pay to get into E+ for non-elites (it could encourage one flight a year to go UAs way versus CO or whomever from non-elites). The "pay to keep it blocked" would never work, because it could keep stand-bys off the plane (not practical).

the language around blocking is "try" not "guarantee".