At the end of the day, I don't think he resigns unless he knows that he'll be convicted in the Senate. And unless there are over a dozen Republican senators willing to put their own political necks on the chopping block by voting to convict Trump, he's not going to resign.

And it won't be because he's worried about impeachment, which is a distinct possibility. That's a likely scenario, but one that still has a ton of obstacles for Democrats.

No, he'd resign because of what's GUARANTEED to happen should the Democrats win Congress: Congressional investigations galore! I mean, a half-dozen overlapping inquiries from every committee imaginable. His presidency would grind to a halt, and he'd know it. And he's the type to blame others and just take his ball and go home.

Trump is too narcissistic and arrogant to leave willingly. He will drag everyone he can down with him before the end.
Will he be removed? I believe so. Before Jan 1, 2019? Doubt it, the wheels of justice move slowly. It will happen.

Or other fatal event.

Outside of that you could have posed the question for 2020 and 2021 as well and the answer would be the same. The votes are not there for impeachment and he is not going to quit.

That way, President Pence will be eligible to serve 10 years. If he's sworn in before Jan 20, he'll only be eligible to serve six years.

10 years of President Pence. Impeachment or resignation of Trump loses some appeal.

this time.