I believe PoS could be used to raise the cost for an attacker further. To date all the concepts I have sketched out have limitations I a find unacceptable but I believe there is a solution. Imagine if someday the hardware cost to attack the network was $5B but it also required another $50B in stake as well.
Here you are admitting that Bitcoin as it stands has a significant weakness.
All things are subject to attacks, to make them stronger one need to be honest about them. Bitcoin assumes that the majority of the energy expended on its POW is not controlled by a byzantine attacker. Maybe there are things that can be twiddled to make the costs of attack higher? But that isn't something that comes quickly or easily most of the times tweaks increase the cost of one already effectively unachievable attack but do so at the expense of opening up a new weakness which is currently absolutely precluded.
Talking frankly about attack costs doesn't mean that anything is weak on an absolute scale. By contrast, the systems where their authors claim there exists no attacks that they can imagine are almost certantly intolerable insecure since a lack of attacks even infeasible ones that we can be comfortable with means that there is either indifference to security, inadequate understanding of their own system, or simply a massive failure of imagination any of which could be hiding quite serious attacks.