The challenge in creating a compelling comic book movie has always been suspending disbelief and making the audience believe that they are witnessing extraordinary feats, rather than cheap camera tricks. CGI has made it possible to realistically display superpowers on the silver screen, a feat that was incredibly challenging to accomplish before high-tech CGI became commonplace. Special effects like those used in Ironman, Thor, Batman V. Superman, and the Avengers films would never have been possible with the technology of the 1980s and '90s. The effects they use and the degree to which they use them is only made possible by CGI technology today. People see comic book movies for the fictional and fantastic, and to see is to believe. The introduction of CGI to comic book movies explains their rise in popularity.
People go to see comic book movies for the character, not for the big CGI fights. Further, Superheroes like Batman and Superman had successful and popular movies in the '70s and '80s in spite of the fact that CGI technology at the time was nothing near what it is today.