It's rumored that Robert B. Parker wanted to kill off Spenser in Valediction (1984) but his publisher talked him out of doing so. I wonder...
I'm not saying Parker should have killed Spenser, but what if he had stopped writing him for a while and concentrated on other projects/possible series (as Robert Crais, Dennis Lehane, and Harlan Coben have done). Then when Spenser returned, fans might be more eager than ever to see him.
If Parker couldn't refresh Spenser with new ideas, perhaps a hiatus from writing him would have done the trick. Of course, it's possible Parker's readers and/or publisher have demanded a new Spenser every year. And it's possible if Parker stopped writing Spenser, he'd have trouble starting up again.