I do notice that as a lot authors grow older and more financially secure that their output slows down.
Martin is the obvious example, but I’ve been waiting some twenty-five years for Clive Barker to finish his art trilogy and the five part young adult series he started in 2002 is stalled at the third book. Neil Gaiman hasn’t put out a full adult novel since 2005. Harlan Ellison, once prolific, seemingly stopped writing in the late 80s or early 90s.
Stephen King would be an exception. Despite being wealthy and “retired” he still pounds out at least one book a year.