>being out-competed by placental mammals every time
Why are they not extinct then? Multiple marsupial species live in coexistence and thrive along with placental mammals all over the world, even in places that have been recently invaded by placental mammals. You wanna count those driven to extinction by humans? as if placental mammals haven't fallen victim to that as well...
I can think at least one predator for each continent that would completely wreck kangaroos, except in europe, but weather alone would do them in there; not all grasslands are the same.