You think Trump is going to resign? Akin to voluntarily Losing? I'd be surprised. Plus it will be so much more fun for the GOP to have to impeach him, for him to put up a fight, for all the indictments to have to come out, for staff, secretaries of various departments, senators and congressmen all have to be carted away in cuffs.
Look at it this way. If he stays president, they can't prosecute him, not until after he is impeached. If he resigns, they can go after him right away.