Re the idea of child-free or non-elite-free flights:

Read today's paper. Now, two airlines will be disappearing soon (until recently just one). The most basic improvement, More Room on AA (appreciated by all passengers), happened only because AA is making tons of money, and they claim their yield management system will keep their revenue the same.

Restricting the availability of seats on a plane (fare inventory is not a restriction towards people) is a recipe for disaster in the airline business -- actually, most businesses. It would only work in a regulated or monopolistic environment.