I don't think a bitcoin millionaire is anything different from a normal millionaire. What if you won your millions in the lottery? Or by trading stocks? Orby selling a company? I would make a difference between all of these. In the end money is money. There so many people willing to sell goods like a car or other things for bitcoins.
As long as bitcoin is valuable, it makes no difference what its value is based on. It is recognized as the same as the value of any other currency in the world and also has a measure of the value of any currency.
When a global catastrophe occurs, not only cryptocurrencies will disappear, but almost all current state currencies will disappear. After all, then no one can guarantee their value. However, it may happen that a catastrophe can only have a man-made character, when for some reason only the Internet and certain types of high-tech communications can disappear. A weapon may also appear that will disable everything that uses electronics. Then paper money will remain, and all non-cash money, including cryptocurrency, will disappear.