It wouldn't have taken "many" years to get another gig, but if he's interviewing for a job next year, he looks better if he can say "I rebuilt Syracuse, Buffalo, Jacksonville, and now want to help rebuild your team" rather than "I had some early success but my new owner fired me. Will you take me?"
And I personally think Pegula is full of . . . . regarding offering him an extension. Even if he did offer one, it probably wasn't a very good offer if I had to guess. Likely one of those offers just so that he can say he gave him an offer (ex: what the Cowboys are doing to DeMarco Murray). It was pretty much an established fact that Marrone and the front office didn't see eye to eye. I really doubt the new owner's first move was going to be to not bring in a new coach and rather retain the coach that the front office dislikes.
Now Marrone gets to work with a young offensive unit that literally has nowhere to go but up (and yet he can still enjoy the rebuilding process that he likes) while being paid like a head coach and potentially increasing his odds of being paid like a head coach going forwards. Also sets himself up to interview for the Titans job if Whisenhunt is canned next year or the Jags job if the offense improves but the defense is lackluster and they fire Bradley.