As far as I know Warren Buffet is against decentralized cryptos so he is not having any crypto currencies I guess.But he still tryint to be rich by using smartness that is suggesting others to don't buy so which makes he will still be the most rich person in this world. Balantly investing on the things based on the price of anything can be considered as fooling but investing on bitcoin can be the best decision of their life time.
A person who has 1000 bitcoins = $5,000,000 will not risk the BIG money. You can retire with $5 million.
Now who will buy 1000 bitcoin at $5000. Those big buyers has purchased 1000 Bitcoins at $30 or $50. That is how ponzi schemes/super-HYIPs collapse/crash. The same is for stock markets and billionaire Warren Buffet (buy buy buy buy)
Still there are many big orders are happening on the exchanges and daily trading volume reaches billions so if you think that no one is buying bitcoin then you are going to lose benefits.The prices can be manipulated by big orders but it is happening everywhere for that we can't just call it as ponzi scheme.