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.

I don't think your reasoning here is sound. PoW is the elected federation of validators, it's just that they're elected in a very flexible way.

Dormant POW could be as much of a benefit as it is a drawback.

If someone owns 10s of millions of dollars worth of miners that are temporarily turned off, do you think this entity would want an attack to succeed?

No. They'd want to protect their future investment in mining equipment. They might just turn that hashpower back in, if an attack was in progress, and take a temporary loss to fight off a temporary attack.

Even the threat of a big player coming in to prevent an attack would make the risk of attacking way to high.

We saw this happen recently when someone tried to attack the Bitcoin Cash network. Defending miners temporarily turned on their hashpower to stop any attacks. And it worked.

> If someone owns 10s of millions of dollars worth of miners that are temporarily turned off, do you think this entity would want an attack to succeed?

Well that depends on how much they're holding and how deep they are in the hole at the given time.

> No. They'd want to protect their future investment in mining equipment.

Unless they were in financial trouble and an attack would get them some cash, fast. The sort of financial trouble that can happen when a company invests, say, millions and millions of dollars into now-unprofitable hardware.

What I am saying is, that it one "desperate" miner attacks, then 5 other miners, who don't want to lose their investment, will defend.

The network is not static. It doesn't matter if one miner wants to make a quick buck, because the defenders have more to lose, and would spin up defending POW if needed.

Until/unless a large player takes this sort of approach, like, say, Bitmain, who have just hit massive financial difficulties.

It absolutely can be in the rational self interest of such players. Relying on other dormant capacity to come up in defence seems fanciful at best.