Or we can admit that vulnerabilities are a likely possibility, despite all of our efforts. Therefore the most secure approach is to understand that limiting the impact of one vulnerability is a reasonable way of dealing with it.
Otherwise you're suggesting running application code as root on the machine isn't a problem, since your application has no vulnerability.
The application should be as resistant to xss as possible but things do sneak through and we should try to limit the damage in other layers.
An example is that you could think you have no xss issues because you use react to do your rendering. Meanwhile you have a window.location = something_from_url which is just as capable of running js code if you’re not careful.
Having the auth (whatever it is) in a http only cookie is one protection. Having it time limited is another. For some applications locking it to an ip address might make sense.
It’s not an either / or thing.