You can't expect that what you've become a master in will keep you valuable throughout the whole of your career, and you want to add to that the fact that most people are now going to be working into their 70s. Being a generalist is, in my view, very unwise. Your major competitor is Wikipedia or Google.
Lynda Gratton, cited in: Shalia Dewan, "Working Nonstop to Stay Relevant," New York Times, September 22, 2012.