So according to you, high difficulty does not provide any protection against any kind of attacks.
For Proof of Stake, difficulty does not translate into security. Difficulty in Proof of Stake is a parameter used to keep the block times close to the target. Security in Proof of Stake comes from the stake. That's why it's called "Proof of Stake".
Try this experiment: pick a Proof of Stake coin like Blade with a tiny PoS difficulty and buy say 10,000 BLA. Now, you'll have some stake. Split it into 10,000 transactions and put these in a wallet on a machine with as many GPUs as you have. Since everyone staking blade right now is staking on their CPUs, the total network hashes is maybe 10 KH/s. So, with a modest investment, you could have easily 100 times the hashes as the entire rest of the network--for staking. Now, see how long it takes you to 51% the Blade network with this setup.
Of you course won't try because you know that such a plan would never work given that you wouldn't own the requisite 51% of Blade to launch a successful 51% PoS attack. Acquiring that stake, my friend, will cost you. But if you try, and when you get close to 51% ownership, please let me know and I will, as a kind favor, sell you mine for market price. Then, when you crash your own hard-won investment just to make a point, we can both have a hearty laugh at your success.
On the other hand, when you get desperate to validate your own ideologies (or more likely get frustrated with me personally for being so correct in these matters), you must resist the urge to set your GPUs solving Proof of Work blocks to attack Blade, because that would basically be your admission that PoW is indeed the weak link in a PoW/PoS coin.