Developers want to change stuff and do greenfield because it's fun, but that's a whole different beast to a career track whose job it is to change things. This goes beyond just poor incentives ["Googler gets a raise because shiny new thing"], into "upper management have created a construct whose purpose it is to disappoint existing customers who like what's already there".
There may be an argument that you'll draw in more people with shiny stuff, than you'll lose when you break old stuff. I only have anecdotes and personal experience that that's untrue [digg? reddit? metro?], and no examples of cases showing that argument to be right.
Also, personally I get my jollies working on other people's old code. I work with several FORTRAN codebases that pre-date my birth, and I enjoy it.