Interestingly,
we've had a discussion about this quite recently (when "heartofwessex" coined the term POO).
In short - yes, pilot only operation is possible on smaller planes, but usually entails two pilots. Most planes require cabin crew and I can't see that ever changing. The basic issues involved here are security and safety, which are more of an issue on an aircraft than on a train. Most pilots are now segregated from the pax throughout the flight, so maintaining safety in the cabin needs cabin crew. I'd argue this is more of an issue in the skies than on the rails, but that's not to diminish the role of the guards. There's simply a lot more than can go wrong on an aircraft and a lot more that the crew would need to do in an emergency.
Besides, airlines have expanded their role to include in flight retail, so they certainly pay their way.