Let's not forget the event that precipitated this topic was the OP, an MVPG, booking a transcon 2 weeks in advance and finding only standard middle seats available. If those routes are so elite heavy that the front Y cabin fills up that fast, who's to say that you'd even be able to get E+ when you wanted it?

If UA could book their flights solid with elites the was AS does on SEA-DCA, the OP would still be in that middle seat. If UA has all those available F and E+ seats available at the last minute, that may be a good thing for you as a elite customer, but for UA it may be a sign of excess capacity. For now, I bet AS is happy to run with high load factors on these highly profitable routes rather than mess with a successful formula.