The real problem is that threat can stay forever if you are unlucky.
I wanted to suggest something about this already, but the game in question was a few versions ago and I was busy modding myself.
Well, here comes my story now:
I was playing a Slavic tribal, started somewhere on the Polish coast and yes I was growing a lot, waging wars after wars.
As I at some point managed to pass Elective Gavelkind (and, interestingly, full status of woman) I was worried when I noticed my realm had grown beyond the size of one kingdom.
So I managed to rush an emperor title after lots of looting, but alas I forgot that vassals could still declare independence.
Well, so I was sitting there, a really old (in his 70s or so) Slavic tribal emperor, and decided it was time to die.
I had been at relatively high (20%+ or so) thread for this ruler's entire life, and had started to accumulate threat even with the one before him. I was planning on having a period of peace now and wait for threat to go away. I was strong enough now to even hold the (luckily fractured) Chalcedonian realms off, and thought I had the time.
But then, my ruler died. Succession, and as I was in Elecitve Gavelkind and my heir not perfect, two of the de jure kingdoms of my empire declared independence. Boom. Suddenly my realm was at half its original size.
Well, time to take the stray sheep back into the herd I thought.
But behold! My threat hadn't decreased at all.
I was still at 25% threat or so, and thus my aunt and nephew decided they would join the existing Pagan defensive pact against me.
So maybe this was unique since I was Elective Gavelkind, but it was definitely illogical to stay at the same threat after succession. Heck, I could have even lost another would-be kingdom if I had been even more unlucky, but in the eyes of the world I would still be the giant oppressor my father used to be.
And if I conquered them again, I would then be at the exact realm size I used to be, but at much higher threat! If this had happened again upon the next succession, I could have probably gone to 100% threat without growing my realm at all.
I was too frustrated by this so I stopped playing this save, but I really wonder now why I didn't bring it up back then.
So anyway now my suggestion is to
a) take independece upon succession into account, and
b) maybe lose a minor amount of threat on succession. Kinda like your vassals inherit part of the opinion they had on your predecessor, but not full.