Characters they can (and will) keep giving sequels to. Holland will be back, Hemsworth has said he'll keep doing Thor as long as Marvel wants him, Cumberbatch also said he'll keep playing Strange.
People think Iron-man and Cap were the cream of the crop. As much as I love those characters (I've collected both for 30 years) no one wanted a bar of them. It was the films that made them popular. Same with any character.
F4 and X-men are vast enough to keep pumping out films for Marvel for the next 20 years (X-men moreso).
This argument that "all the good characters are gone" is total bs.