But if an attacker obtain 51% mining power in a PoW network, there's nearly nothing you could do, other than go out and out-spend the attacker by buying more hardware and spending more on electricity. I'm sure the hardware vendor and electricity companies would love to see this happen. If no one step up and out-spend the attacker, the attacker with their vast amount of hardware can PERMANENTLY disable the Bitcoin PoW network, there's nothing you could do to stop them, other than... converting Bitcoin to PoS 
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.This is false, I can come up with two solutions quickly without even trying:
There are solutions to modify the protocol to defend against Bitcoin after a 51% attack just as their are ways to mitigate DDOS attacks. One solution would be to temporarily identify the IP ranges where the attack is coming from and block those ranges, followed up by using anonymous nameIDs associated with miners that have reputation rates where the network could purge hash values from attacking miners.
Another solution is to create a PoW/PoS hybrid.
It seems your judgment is clouded as you aren't critically analyzing the strengths and weaknesses of PoS vs PoW and are merely trying to market Nxt/BTSX.
If this isn't the case than can you name several security flaws within Nxt/BTSX that Bitcoin doesn't have or do you claim that PoW offers no security benefits above PoS?
Your 1st solution doesn't work, it's trivial to change IP.
Your 2nd solution is what I already suggested at the end: "converting bitcoin to PoS". A PoW/PoS hybrid is still means converting at least part of the network to PoS.
I do think PoW has no security benefit over PoS, otherwise why would I be calling for an end to PoW, and favoring PoS?
I'm not marketing NxT or BTSX, actually I'm one of the biggest NxT hater there is, I have already shown you my opinion against NxT in previous threads. I own 0 NxT, and a small amount of BTSX. My Bitcoin stake is 100X more than my BTSX stake. I would not mention NxT or BTSX if I didn't have to, but it's hard to discuss PoS without mentioning these existing PoS networks.