I see your point, I was wondering if that was what you meant, but I don't agree with it. Apart from anything, it strikes me that if you stick to this principle then this whole website is a little redundant - we can't predict how history will go beyond the lifespan of those alive at POD, and even then after a few years our ability to read personalities fade in the light of unanticipated events.
I don't mean offense, and it suddenly strikes me that without voice tones it probably seems that way, but I never quite bought into that idea all and every historical figure should be butterflied away if born after POD. To me it kind of seems to make all the talk about alternate history a bit pointless, at least past a few decades.
But if we go the other route and think "How will a Stuart Monarchy act when Hitler takes over? Or when Princess Diana dies?" It seems incredibly stupid to think things will get that convergent.
Its decent to take the middle route. The Buttefly effect can keep things farmiliar enough to be recognizable and yet different enough to be interesting. Also its all about likelyhood. If William living causes any differences in the myriad of wars until 1763, then France may not be able to take Corsica. That is gonna butterfly away a French Napoleon no matter what. Now he could have a different name (Same father and mother but different time of conception) and go on to be leader of Corsica or some other sufficently cool Italian state. Or he could go to Britain and become cooler than Nelson, but the reality is that it makes it all the more interesting to have this happen, so the butterfly effect makes alternate history not pointless but incredibly interesting.