I used to be confused as to why WB didn't let Singer have another go. SR
did not less than decent box-office, and many superhero sequels improve on the first movie's take anyway, so why not?
Now I know.
I really doubt this was why WB said no to Singer. It looks to me like there would have been PLENTY of room in this story for the kind of action and excitement the studio wanted.
More likely, they just realized that there wasn't enough enthusiasm out there for Singer's version of Superman, and it would be best to just try again later on with a new director.
As much as I loved SR and wanted a sequel... I kind of have to agree with their reasoning.