Essentially, Will and Steve are friends in real life and Will knew that following in the foot steps of Steve was basically setting anyone else up for failure. He called Steve and offered to be the one to fill his shoes to basically make the transition away from Steve more smooth and viable from a commercial standpoint point. Some random couldn’t replace Michael Scott, but everyone would totally step in to watch Will Ferrel give it a try.
When I hears that story, it changed my outlook on his character. I didn’t like his character at all, but I respect what he did for the show.