This would be reassuring, but are you sure? I seem to recall seeing the Game Over Imminent warning appearing at the top of the screen, and I did have other dynasty members in existence.
I've come close, but never lost the direct thread of immediate dynasty succession. It's not game over if you have any dynasty holding land anywhere? I always thought it had to be dynasty members of one degree of separation, to something like that.
I'm not sure of the exact rules. With smaller realms, I always found elective to be more unpredictable, but in the HRE it's laughably easy to pick and choose your successor.