More and more gaming leagues are dropping support for 1.6 and moving onto Source, the same goes for the players I think. Nothing is really stopping you from mapping for the Goldsource-engine for the rest of your life, the question is if someone else than yourself will be playing your maps. And if there is none to appreciate your work I think you'll soon get tired and move on...
Myself, I will probably continue to map for GoldSource for about another year, then I'll move on, not sure if Source will be next, probably some other engine...