That's possible. But that doesn't mean a lot of voters won't become convinced he's guilty.
He has no guarantee of being elected and even if he were, the SCOTUS will likely find that he does not have the power to pardon himself. He might be allowed to put his trial off till after his term has expired but then he is almost guaranteed to be impeached again with what will very likely be a Democratic House and Senate. Alternatively, his cabinet might pull a 25th Amendment on him on the grounds that he's too involved with his legal problems to do his duty to the Constitution.
If Biden declines it will be because of the way Trump behaved at the 2020 debates. If Biden does not decline, Trump will.
Trump refused to appear remotely at the second debate in the 2020 election and his obscene behavior at the first debate showed quite clearly that he never had any intention of actually debating Biden. He was scared shitless of having to actually answer a question.
So, why isn't Trump debating Haley? Are you afraid people will come to the same conclusion you say they'll come to with Biden? That he's afraid? And that his populariy will plummet?