I think POW guarantees security based on economic assumptions. I think those economic assumptions as you say, only protect against the obvious brute force attack. I think true security requires looking at things statistically and no I do not think POW is more secure than hybrid POW+POS, not is it more efficient for stakeholders. What I mean is POS provides for more evovability in governance than POW.
I could go in on this topic but I would rather let Dan communicate on the technical side of things because he is one of the main innovators in POS. My opinion is that while I like DPOS, I have my own model of how things can or should be, based on what some people might think of as "sci fi" if I tried to describe it today. All I can say is what looks like sci fi if described right now will not look like sci fi in 5 years, and all the topics I discuss are in development, even if we do not see the fruits for up to 5 years.
The botnets, the agent based ai, the post I wrote on autonomous negotiators, these will be in the crypto-space probably in the next year or two. AI is coming and will merge with blockchain and this in my opinion is unavoidable because they have so much synergy and will make trillions in profit.
I'm very interested to know about your "sci fi" model.
AI is certainly begging to be connected with the blockchain. But the development must be kept open source. There could be private implementations but closed source AI could be worse than a FEDcoin.