It's not as if these people are just sitting on it, not spending it, either. At least in the US they get taxed out the ass for their worth, which strongly encourages donations back into the economy to receive some tax breaks. Quite frankly, your wealth will run dry if you just sit on your ass not spending anything or trying to earn it back in some way, regardless of if you can live off the interest from it or not. Inflation/taxes will break that.
Anyways, I'd like to hear your thoughts on some of the following:
-1 billion what? Currency wise. Euro vs yen is obviously a HUGE difference. Need a base.
-1 billion what? Net worth wise. People that break the billion mark and are forced to spend it will have roughly the same amount in their assets as they did in more liquid form (cash/bank accounts/etc). Therefore, what they buy could simply be bartered with to avoid spending cash in the first place.
-Inflation. It's inevitable, since scarcity rules over economics. Natural resources are limited, and as those run dry, things will become more expensive. Eventually (~1-2 hundred years, assuming humans are still around), the ability to have 1 billion dollars would become about as common as having 1 million. Would the cap be able to rise?
-Poor people. There's no way to guarantee the money that's reinvested by wealthy people to hit the pockets of the poor without regulation. In which case, it truly would be a communistic thought process.
There's more, but I'm already getting too serious about this. I'll worry about direct economic effects later. >>