For example, there is - in my opinion - a low chance that the Gates Foundation will expend its resources within 20 years post the death of the last of the two.
They are likely to have something total equivalent to (in present dollars) perhaps $250-$300 billion to get rid of in the next 40-50 years. I'd be skeptical they can get rid of it that fast in their model. They're eroding that mass of capital (presently near $190b) so relatively slowly that by the time Gates is 80, they'll probably still be dealing with $200 billion in today's dollar.
A succession of massive, but formally time-limited foundations, administered in lockstep by dynasties of custodians would add a very interesting touch to any future scenario of fiscal neo-feudalism.