Very much possible! Though 21 million may sound just a little, not more enough for like more than 8 billion people globally, but what makes it possible for it is that, unlike the fiat currency where the government can print as much paper money as they want, bitcoin is limited in supply minus all those that lost in circulation. With that said, bitcoin's value in the future may be 10-20 times higher than what it is today and that means the smallest fractions of bitcoin today will be too much in the future for an individual to hold. No matter how much bitcoin the whales are hoarding, I don't think the total percentage of whales who held large amount of bitcoin are close to 50%? (not sure about that though). So, it's not something we worry about, not everyone are interested in bitcoin though as there are still a lot of people who prefer to use the traditional banking system and online banking service.