Sure, the protocol will adjust for the falloff in hash-rate as miners ditch their hardware- a perfect opportunity for a malicious entity to buy up their hardware and use it to attack the network.
The world is filled with situations where something with lesser economic value secures something with higher value. That's a sign of efficiency.
My house is worth $250K. But you couldn't capture that much value by breaking in, so I haven't spent $250K on a security system. If I did need another $250K to secure my first $250K investment, it wouldn't be a great investment.
Suppose it only takes $3.5B to attack a $7.3T network. So what? Unless your plan is to burn billions of dollars for funsies, you'll need to capture at least $4B worth of value for that attack to be worth it. You think you can find someone to anonymously make a $4B crypto sale to you, who won't know where to hunt you down when you try to 51% away the transaction after the next block in 10 minutes?