Price goes up due to demand. at $15 million it wouldn't be a few people who own Bitcoin, EVERYONE would own bitcoin. Not sure you understand price dynamics. If few people own Bitcoin price is low. More demand higher the price goes.
First of all I do not agree with 15 million dollars price neither, that sounds absurd and definitely not realistic at all, it is probably just made up number to get attention. However just to make it easier for people to understand how people could own bitcoin at that stage, there are two methods that you can assume.
Let's say there is bitcoin price at 15 million dollars, how about buying 100 satoshi instead of trying to buy 1 bitcoin? Or get 10k satoshi? I mean people could try to buy bitcoin according to how much money they have, and even 1 satoshi is a decent amount in that situation and would be an investment, so you could go with that. Or the second method, do you really think bitcoin becoming 15 million dollars would mean all the alts would be current price? They would go up too, and you can try to buy 0.1 ETH for example? Or whatever coin you want, whatever amount you want? That would be an option too.