I'm a big comic book reader and I know that there is a decline too in sales.
Comic cons do not rely anymore on the comic books, they rather rely on guest star appearances from tv shows and films but its not the same.
In books, I read only when there is a need to read one, which happens not so often.
It's no surprise that big publishing companies invest in celebrities more than classic book authors.
For example if Tom Cruise decides to write a biography , publishing houses will battle with each other like wild dogs on who will promote it first.
Celebrities have thousands even millions of followers and that means big business.
My first book I will self publish , at least this way it will be out when it needs to be out.