Nice list. Now you need a RCE exploit and a chained breakout exploit thought. That's a lot of cash.

Given this and that the process isolation also protects against meltdown/spectre type attacks, I think we can agree that this type of fine-grained sandboxing is a requirement for secure software, no?

However, next to no software is using fine-grained sandboxing. From the top of my head only qmail, djbdns and gatling come to mind, none of them are for end-users.

So what end-users software does actually approach or surpas browsers in this regard?

Then lets not pretend that a browser is sufficiently secure for people considering using OpenBSD. Given the fact that by its nature a browser runs untrusted unreviwed code on your device it does a pretty good job of making it difficult to exploit, but it is irresponsible to say that its sandboxing cant be bypassed when clearly it can.