Depends on the mission, the end goal (if any), as with so many other things.
If you're Gates, you likely feel an immense obligation to not destroy $100 billion (as with Buffett), as it's meant to be put to use as a public good. That has also become his professional life. And ideally you conservatively grow it further, to be able to bring that many more resources to bear on extremely expensive problems.
This is why my respect for Bill Gates has gone up so much over the years.
It’s cool when folks like Warren Buffett pledge to give away their personal fortunes when they die. However, his personal sacrifice is actually quite small, as he only gives it up when forced to by death.
Bill Gates, on the other hand has two valuable resources to offer and he is actively giving away both right now: 1) much of his vast fortune, 2) his time, expertise, and influence to champion causes for the greater good.
Buffett doesn't exactly spend it now (he's got famously cheap tastes). He's also giving much of it away while still alive: $3bn more in Berkshire Hathaway shares to the Gates Foundation last year. Forbes claims that's 40% of his BH holdings given away so far. http://fortune.com/2017/07/10/warren-buffett-donates-gates-f...
If you're Gates, you likely feel an immense obligation to not destroy $100 billion (as with Buffett), as it's meant to be put to use as a public good. That has also become his professional life. And ideally you conservatively grow it further, to be able to bring that many more resources to bear on extremely expensive problems.