This is a somewhat weak argument. I could say the same thing about door locks.. no lock is perfect and an attacker with sufficient resources can successfully subvert pretty much any lock. We don't depend only on locks but on police, neighbors, security-alarms, etc. This does not mean that locks are useless.
PoW provides some amount of security but in order to pull off a double-spend you have to not only gain majority hashrate, but also somehow actually spend/cash-out both 'copies' of your coins. This carries substantial risk as you first have to be long those coins (not smart when you are attacking the network). Then convince exchanges to convert both 'copies' while hoping they don't have systems that notice the massive increase in hash-rate nor the unexpected fork you are creating.
Simply shorting a coin, then attacking as more of a DoS may be more feasible but still carries risk as a successful defense could positively affect price.
PoW provides some amount of security but in order to pull off a double-spend you have to not only gain majority hashrate, but also somehow actually spend/cash-out both 'copies' of your coins. This carries substantial risk as you first have to be long those coins (not smart when you are attacking the network). Then convince exchanges to convert both 'copies' while hoping they don't have systems that notice the massive increase in hash-rate nor the unexpected fork you are creating.
Simply shorting a coin, then attacking as more of a DoS may be more feasible but still carries risk as a successful defense could positively affect price.