It is kinda interesting to toy with the idea of the theoretical limit of crypto. It has to do more with mining and less with how many millionaires are on Earth. If the reward is high enough everyone and their mother will mine coins. etc.etc. I may look into it, but until then just keep in mind, the ATH has already been reached and will never be surpassed again.
There was a reddit thread about this same topic with a lot of real bad assumption about market caps. This post is relevant though:
"You can’t talk about theoretical maximum for Bitcoin without talking about energy expenditure. Currently Bitcoin network is consuming as much as Argentina, or 0.5% of the world energy. If you hope that the price will go 10x, then Bitcoin would use something like 5% of the world energy. US is consuming about 20% of the world electricity. So Bitcoin increasing 40x in price would get us there. Obviously this can’t go on for ever and at some point we will have a strong reaction from media and politicians against absurd usage of energy for almost no social usage. Especially when other technologies like Ethereum will have moved to proof of stake and consume then significantly less energy while offering much more socially valuable use cases and scalability."
Another good point:
"Interesting nobody's considered transaction costs in this analysis. If Bitcoin hits $1,000,000. each, a Satoshi will be equal to a penny. Not a big deal, except the fastest and cheapest transaction fee is currently 102 satoshis/byte. For the median transaction size of 224 bytes, this results in a fee of 22,848 satoshis.
Thus, the average transaction will cost you $228.48. To get a transaction fee of 3% (say what Visa might charge) you couldn't buy anything less than $7600.
Sure, prices could drop, but can't really get below 1 satoshi/byte giving you a mininum cost per transaction of $2.24. Now you need to be spending at least $75 to get a transaction fee below 3%."
TL;DR: Bitcoin at 1 MM is unusable as currency.
