I disagree with your first point. If you have 51%, you can do various attacks that won't crash the price(which actually may not matter if you are in for the long term). This is why I like dPOS where the control is in 21 actors. In Bitcoin and Ethereum, 5 pools basically decide everything.
As an example of attacks, you could target other miners: build your own fork but include all the transactions from other forks. This means that you are not going for the double-spending attack (which is not my favourite attack actually) but you are going for the mining rewards. Users are inconvenienced but they don't loose money.
For the whole cryptospace, the N°2 should be the most concerning. With all the money and people involved, did we achieve anything? I do believe that cryptocurrencies and blockchain are not just a technology but more of a movement. For me the success is not to replace everything with cryptocurrency, blockchains, token, decentralization... The success will be to put pressure on centralized systems, so that they give back more power to users or rethink their models.
A single person can own multiple bp nodes.
I agree. Comparing to PoW, it's the same as a pool having different identities.
I don't think you can do better than making it easier to have more block producers/miners.
If you're right, we'll see Bitmain do that, perhaps.
Who knows?! Coming from security fields, I am more paranoiac than most. I am wondering now if there were a massive crackdown at cryptocurrencies, who will survive ? I would bet GPU-mined coins would die first because you can re-use them.
Is your attack transparent or hidden? How does the market respond?
The attack should be transparent enough which is why I try to spare the market by re-including transactions from concurrent miners. But I have no idea how the market would respond and I don't think we saw this attack live.