In my opinion, hijacking is impossible. Blockchain technology is not easy to hack or hijack.
The y cam attack the market though, they buy every coin available in the market and hodl and let it sit in there laps never to see the light again or they will buy all the supply then continuously dump and pump the market to the point that retail investors can't keep up.
Do you have any idea how much money they would need to buy every single coin and let it sit idle in their wallets? They would need trillions of dollars, tens of trillions of dollars. It is not possible to hijack Bitcoin just by buying the coins in circulation.