The Federalist Party basically died with Hamilton. The Communist Party of the USSR was thought to be entrenched right up to the moment it wasn’t. Political change can happen surprisingly swiftly.
That goes double when the group is rallied around a specific person. That person goes away and the group shatters. The GOP is primed for this, and we’re somewhat fortunate that even in the worst case scenario, they rallied around a 77 year old man who eats a lot of McDonald’s.