I can, and I would.
> or virtual machine-based isolation
A little different because a VM can be designed to offer a rigid security boundary (with a solid model behind it) rather than as an ad-hoc mitigation technique.
> So the conclusion is all mitigations are completely useless? If it's your opinion, I'm fine to agree to your disagreement, many sensitive C programs need to be written in a memory-safe language anyway. But as I see it, as long as there are still C programs running with undiscovered vulnerabilities, and as long as attackers have to add more and more up-to-date workarounds and cracking techniques (ROP, anyone? but now the most sophisticated attackers are moving to DATA-ONLY attacks) to their exploit checklist, then we are not losing the race by increasing the cost of attacks.
> The principle of Defense-in-Depth is that, if the enemies are powerful enough, it's inevitable all protections will be overcame. Like the Swiss Cheese Model (https://en.wikipedia.org/wiki/Swiss_cheese_model), a cliche in accident analysis, eventually there will be something that managed to find a hole in every layer of defense and pass though. What we can do, is to do our best at each layer of defense to prevent the preventable incidents, and adding more layers when the technology permits us.
> For real security, I believe memory-safe programming (e.g. Rust), and formal verification (e.g seL4) are the way forward, although they still have a long way to go.
I think the defense in depth / swiss cheese approach has shown itself to be a failure, and exploit mitigation techniques have been a distraction from real security. It's worth noting that systemd is both recently developed and aggressively compatibility-breaking; there really is no excuse for it to be written in C, mitigations or no. Even if you don't think Rust was mature enough at that point, there were memory-safe languages that would have made sense (OCaml, Ada, ...). Certainly there's always more to be done, but I really don't think there's anything that would block the adoption of these languages and techniques if the will was there.