I think the future is just fine. The predicted oversupply of RadOnc docs has turned out to be a shortfall instead, so the jobs are there.
Compensation will probably decline, but this is no different than every other medical field out there. Manufacturers of new tech are highly aggressive (some would say too much so) in getting their treatment machines approved by insurance -- sometimes w/o good evidence-based research.
Finding jobs in large cities is very tough. It is often times highly situtational -- for instance, you may be lucky enough to graduate in a year where a given city has a lot of vacancies. If you go to a "big name" program with a lot of contacts and pull, this might open up a few doors for you but it is far from guaranteed if you want to mork in major metropolitan areas. For most people who want to work in big cities, academics is the way to go.
Relocation is really up to you. Some continue to work in the same city, many others move elsewhere (often multiple times).