I think he made most of these deals in the late 1980s and early 1990s (he wouldn't have been hard up for cash at the time, but the big checks for the film rights certainly would have been enticing). It was after the films started coming out that he took a hard line against film adaptations of his work.
Then he shouldn't have signed on the dotted line. The fact that he did means he has no right to bitch about the situation now (or more specifically, of course he can bitch all he wants, but nobody needs to take it seriously). He was greedy, he took the money and lost control over the property. What a crybaby.
If the movies are somehow inhibiting sales of the graphic novels (because they are bringing the novels into disrepute), then that's something for the corporate bean counters to take into consideration. But I sincerely doubt that sales of graphic novels could ever compete with sales of movie tickets, plus movies are going to be such a publicity boost for the novels that even if the movies suck, they will drive graphic novel sales up.
I suppose in theory there's a case where you would hold off on doing a movie for fear of hurting graphic novel sales (or real novel sales) if the movie sucks but I have never heard of suck a case happening in real life and would be astounded if the economics of it ever made sense.