They don't say "we should replace capitalism when stumble upon a better way of organizing human economic activity". Which is a pretty bland statement everyone agrees with.
They said we must replace capitalism because of the horrors it perpetrates.
That is a very strong statement.
But I guess they could be saying "we are morally compelled to replace capitalism with a system that may or not work exist that may or may not solve the problems we have attributed to capitalism without showing that any other workable system fixes these problems, only that they happen to exist under our current system."
Which doesn't strike me as the most useful criticism of capitalism.