How was he preserving himself by resigning, as opposed to threatening to take everyone down with him?

Take down who? The repubs apparently weren't willing to go along with him- certainly saved him from impeachment

If he had fought - like Clinton or Trump shamelessly fought - he could have made it extremely ugly and costly for every member of his own party who opposed him.

I don't remember that my parents' republican friends and family were willing to back him up- it took quite a while for him to rehab himself