And how tenable is this distinction now that a third space agency carries out manned flights (China), and others are poised to follow?
I can think of no other profession/job title where this distinction is made. Also, with the increased cooperation between spacefaring nations, the lines are getting blurred: what do we call an American who's employed by NASA, but is trained in Russia for a flight to the ISS aboard a Soyuz? Is it logical to have some ESA personnel be called astronauts, and others cosmonauts?