First, Reeves was never that popular an actor. I saw one biography list him as just good enough to star in B-grade movies or have bit-parts in A-grade movies. He did not support himself solely as an actor. He also dug septic tanks and wrestled. In fact he took the Superman role because it was the first steady acting job he had landed in a while.
The problem with him moving on wasn't so much that he was identified with the role. It was that he was identified as a TV actor at all. In Hollywood at the time, TV actors were second-class citizens. It was rare for an actor to make the leap from TV to movies and a movie actor who had done TV was on his way down.
It was also a transition period in acting. Reeves was an old-style actor as opposed to the newer method-actors.
Reeves must have know all of this so his lack of success in landing follow-up roles should not have been a surprise. It is possible that he had been fooling himself but there was more to it than simple type-casting.
In a related issue, I saw one reviewer take a swipe at Reeves' acting ability based on Superman.
The movie stops short of asking whether Reeves was much of an actor. He certainly wasn't much in his most famous role, playing Superman and Clark Kent with exactly the same voice and swagger. It took Christopher Reeve to show how well the parts could be played by taking them seriously.
This is unfair for several reasons. It is always unfair comparing a performance on a TV series to a movie. TV shows are under pressure to get the scene shot and move on. Movies can take much longer to get a scene right. This was especially true of the Superman TV show which was often shooting multiple episodes at once, doing as many as scenes from three different episodes in a single day. The characters normally wore the same clothes in every scene to simplify continuity.
More important, the producers decided that, since Clark Kent would be on-screen more than Superman, he had to be like able. They chose to have both characters acted the same. Smallville made a similar choice, making Clark a football star instead of an equipment manager.
When Christopher Reeve was filming Superman, Clark was given a much smaller role. He is role is mainly as a foil for Lois. Superman has more screen time.
The producers of the TV show made a wise choice. The George Reeves version of Clark Kent is someone you would like to work with. His Superman struck just the right tone. The Chris Reeve Superman was always just a little too good to be true. It is enough to rescue Lois. He didn't really need to tell her how safe flying normally is.
Superman returns is little more than someone else doing a Christopher Reeve impression. It has no new insights.