he won in 1972 with a majority something like 63%. So he fell a lot farther, and very fast considering. That's something to keep in mind-- even with Nixon, it was more than 2 years from "crime" (the break-in) to his resignation. (Also, let's not forget that his vice president was indicted and resigned in 1973-- we could only be so lucky.
Nixon was a crook and monster, but he had a real care about doing some good things (along with doing bad things). He had a reason to be president (going to China, starting the EPA) besides just being acknowledged as the biggest bestest ever. So when he started losing power and influence, he didn't see much reason to hang on to be impeached.
But, as you say, Trump could just quit doing even the minimal work he does, keep tweeting, claim credit for anything good and blame Obama for anything bad, go golfing a lot. He has no need to be any good at this, and no agenda to get done. If Barry Goldwater's ghost showed up and said, "You should retire," he'd probably say, "Why?" It's not like he cares about anything getting done.
I do think he could feel aggrieved enough to take his ball and go home, and he might be willing to make a deal (as Agnew did, and Nixon probably too) to avoid prison. But otherwise, I think he's going to cling to the position. In that case, I can see Pence (whose raw ambition is surpassed only by his ruthlessness) deciding to get rid of Trump-- 25th Amendment or something. And of course, Mueller could find the goods on him and the Congress could impeach.
But Nixon had some pride-- not just arrogance, but pride. I can't believe I'm saying this, but there's someone worse than Nixon, and more important, he's shameless.