So blindingly obvious, now that The Cranky Flier blog pointed it out:
There is value in the upsell.
When American first rolled out increased legroom for all coach seats on the airplane many years ago, the idea was to become a premium airline that people would pay extra to fly. That didn’t happen. A lot of coach travel is viewed by consumers as a commodity. They just want to get places for cheap. So when American removed seats, it increased its unit costs (costs ended up being spread out over fewer seats) and it couldn’t get enough revenue to justify it.
With United, however, Economy Plus was just a subsection of coach. It took the airline years to figure this out, but there are absolutely some people who will pay for more legroom. Some people. United’s Economy Plus has now become so popular and profitable that not only did the new (ex-Continental) management decide to keep it despite biases to do otherwise, but American and Delta have all copied it as well.