Trump needs to stay on as president* or else he gets stuck in a cat5 legal shitstorm
even if he cuts very artful deals for federal pardons covering everything he ever did, there's still plenty of state-level prosecution he'd be running from
even though he's polling at a disadvantage now, he can still win by cheating. he just has to publicize some semi-plausible victory scenarios for his supporters.
if he resigned the presidency and left the race for reals, GOP enthusiasm would crater hard. i do believe his support among Republicans remains quite high -- 80-90%. that's a lot of potential hemorrhaging for the replacement candidate to try to shore up while not losing the very voters they'd be trying to gain back.

basically, Trump has put the GOP in a stupid bind of their own stupid making. they're stuck with him now.