one bug is all it takes

In theory.

But real defenses are generally multi-layered. And in that context, a Swiss cheese slice with only one hole is still extremely valuable.

Well, that's where OpenBSD falls short, it lacks facilities to really enforce defense in depth - even NetBSD has some better features in this regard.

I don't think that's true? It runs most services as their own separate users, with pledge+unveil to limit what they can access even more. That's very much depth.

LPEs do need to be fixed, but for most people it's not a threat model they need to worry about.

this is a misconception

yes, most company settings don't run untrusted code, and OpenBSD is mostly used for servers not employee devices

but that doesn't mean LPEs aren't quite relevant, because they matter for pretty much everyone if combined with other vulnerabilities, like RCE, supply chain attack etc.

and while RCE are becoming less common, supply chain attacks have been increasingly more common