Bitcoin's protocol doesn't just rely on the idea that no one will get 51% of hashpower to avoid getting attacked, the point of PoW is to make attacks so expensive that it would deter attackers from doing it. So, theoretically miners could try it, but it would be hard to profit from it, and it won't be sustainable in the long run. Also, from the point of view of miners, it's like killing your goose that lays golden eggs - if their attack is successful, it would cause panic and drop the price, thus reducing their profits from mining.