I think one thing to consider is that there is economic disincentive against acquiring >50% of the network, and even more disincentive against attacking it once

I think one thing to consider is that there is economic disincentive against acquiring >50% of the network, and even more disincentive against attacking it once you have 51%+. You have to pay full value to get >50%, but Ethereum could lose absolutely all value if anyone gets to >50%, so whatever the investment to get that stake would go to zero. The issue is whether can malevolant entities or coalitions can reliably be detected.