you are describing a core function/innovation of bitcoin. the mining difficulty adjustment

btc price goes down -> not profitable to mine -> difficulty goes down -> not worth to attack

btc price goes up -> profitable to mine -> difficulty goes up -> not worth to attack

bitcoins price in usd has to double every 4 years to keep the hashrate (napkin math) which it easily did in the past. i guess you're assuming (based on what?) it won't in the future