There's "ending on a high while your show is still good and popular" and "pissing people off by refusing to produce more of something that is very much in demand".
For the latter, see Sir ACD's writing career after the Final Problem.
Even if Sherlock is very very dead looking, stuffed into a drawer in St Bart's morgue, brought out and partially disected by Molly then John takes his skull to keep on his mantlepiece and talk to, I'm not buying it. There will be more.