Alot of their "original" work still exists in the special editions, especially in Episode V. As for Kersh, he knew what he was getting into when he directed Episode V. You don't think he saw it as an opportunity to further help his career out if he was a hired hand in the biggest sequel of all time at the point?
And Lucas did produce (executive considering it's his money they're using) and write Episode V. Episode V doesn't work without Lucas' draft after Leigh Brackett died.