If it were guaranteed that the price of bitcoin would stay the same as it is today, would you still buy it? My guess is, the answer is no. But if its value lies in being a currency, price stability is essential. And if your answer is no, how is it then not just a Ponzi scheme? In other words, everyone is buying it on the hope of someone paying more for it in the future, not to use it as a currency.
I'm sure that a significant part of people invest in bitcoin in the hope of making a profit by increasing its value. If they knew that bitcoin would not grow in price, they would not be buying it. Now it is still difficult to judge what will happen next with bitcoin, because as a payment system it does not show its simplicity and reliability in use. Bitcoin does not yet have a developed infrastructure, and therefore these two factors greatly influence the fact that it spreads very slowly throughout the world.