If Starmer appointed Mandleson without knowing the contents of those emails (or similar information collated by HOLAC) I'm not sure what he's done wrong.
If he appointed Mandleson and did know the contents of the emails (or could have reasonably been expected to know based on the information provided to him) then I'd imagine he'll have to go.
Knowing that Mandleson was a money and power obsessed shit isn't really sufficient in itself.
As with most things we'll have to wait for the information.
Waiting is much less exciting than deciding in advance I grant you.