21 million BTC 7.5 billion humans = 0.0028 BTC per person. If you control more than .0028 BTC, you will eventually be above average wealthy.
i would not counts every human, between them there are ill people and 0-3 years child, do you really thing they care about bitcoin
so it's more like the double of that, we can round it to 0.006
Another problem is that bitcoins can't easily generate income yet. So you can't easily earn interest on your stash of BTC to live off. You have not choice but to slowly sell them to fund your wealthy lifestyle, not a good option.
Bitcoin Rich (today) would need at least a couple of million USD in bitcoin.
i don't see a difference with fiat, you earn bitcoin fairly easy without any regular job like with fiat, so it is much easy to earn bitcoin than fiat
2M is relative, it depend how many money you need to consider yourself rich, personally i would consider myself rich even with 500k, but better with 1M, more than that it's pointless for me