He doesn't want jail time for him or his children and he doesn't want his Trump brand exposed as a criminal syndicate. Beyond that his narcissism won't allow him to become the first American president successfully impeached AND removed from office, not without a no holds barred fight anyway. Beyond that Trump might in some manner offer to not run again in return for allowing him to finish this term without facing prosecution, now or in the future. Maybe he would hand off the presidency to Pence during his last week in office in return for a pardon by Pence - something along those lines. Obviously this is speculation, but I agree with the bottom line you stated. If he doesn't get to sign off on his exit strategy he will welcome a complete constitutional crisis. That is Trump's bargaining chip, with the threat of Cliven Bundy type "uprisings" looming behind it.
Republicans are terrified of breaking ranks with Trump unless they are prepared to totally walk away from politics after doing so. Like you said, the legislative branch of government has become non functional as a check on Trump's executive power. Unless Democrats do the previously unthinkable and pick up at least three seats in the Senate in 2018, even with the playing field dramatically skewed against them, Trump has a reasonable chance of keeping enough Republicans in line there to prevent the super majority vote needed to remove hm from office. A three seat Democratic pick up in the Senate would mean holding onto all of our incumbent senators up for reelection in red states PLUS pickups of seats in Nevada, Arizona, and Tennessee PLUS either a second seat n Arizona if McCain vacates his seat OR a Senate win in Texas. An anti-Trump mandate of that magnitude might be needed to convince enough Senators to move against him.
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