An entirely plausible scenario resulting in a situation in which Trump refuses to leave office, is that something stupid fucky happens with the election and the results are disputed.
What's the mechanism for an election where the results are disputed? Fraud.
All we need to see for a nightmare scenario is a few key counties in toss-up states where the results are wildly off the historical patterns or exit polls, and when a recount is ordered we find out the votes that were stored electronically have been corrupted. It doesn't even have to be an actual attack, it could just be a bad machine in the wrong place, and suddenly no one trusts the results.
Look at this thread. Trump wins a historically blue county by a dozen votes and the machine breaks and there's no papertrail, who here is gonna take that as legitimate, even if it is?