Not everyone will get 1BTC even today when a millionaire can still consider it cheap. There is always going to be someone who will sell though. A fraction of BTC is still BTC or there is always ETH or whichever altcoin is preferred. It's a free market where anyone can buy, I wouldn't be worrying about the millions of millionaires if they are not buying BTC, their $millions is not mine anyway.
Bitcoin is Bitcoin, buying it you can feel confident that everything will be fine with your coins. And at the expense of altcoins, these are always increased risks, what risks they are exposed to, I will not enumerate all of them well know. Well, if you look at history, altcoins almost always get cheaper relative to bitcoin, so the conclusions are obvious.