what I think they meant by that, was that as you progress through the game, you do not work on every hero because you get better heroes to replace them.
Examples:
Yax -----> Miguel
Frozone -----> Olaf/ Elsa
Elastigirl/ Vanelope -----> Merida/ Mike
Ralph -----> Shank
And so on…
But hey, that is just imo…
and I am not sure that is what they arw doing anymore becuase they have added Hero refreshes. But if it was, it woulf explain why thr new heroes are more OP than old heroes.