You can only understand this if you give up thinking in absolutes. If indeed nobody wanted to sell, the price of BTC would go up to infinity.

What actually happens is someone decides that is high enough and takes some profit. The bid price has to go up high enough to meet the lowest ask. Upward jumps in the price of BTC happen when there is nobody left to sell at the current price, and the price has to jump up to the next ask. Downward jumps happen when nobody is willing to buy at the current price and the price has to fall to meet the highest bid. With enough players trying to time the market, the market remains fairly liquid and the price doesn't jump too massively.