Comment by Spivak
This doesn't actually seem that bad to me? Browsers don't let random pages on the internet hit localhost without prompting you anymore so it's not like a random website could RCE you unless you're running an old browser—and at that point that's the browser's fault for letting web pages out of the sandbox. You shouldn't have to protect localhost from getting hit with random public websites.
The rest is just code running as your user can talk to code running as your user. I don't really consider this to be a security boundary. If I can run arbitrary code by hitting a URL I accept that any program running as me can as well. Going above and beyond is praiseworthy (good for you turning on SELinux as an example) but I don't expect it by default.
> Browsers don't let random pages on the internet hit localhost without prompting you anymore
No, that's a Chrome-specific feature that Google added. It is not part of any standard, and does not exist in other browsers (e.g. Safari and Firefox).
> The rest is just code running as your user can talk to code running as your user
No, that assumes that there is only a single user on the machine, and there are either no forms of isolation or that all forms of isolation also use private network namespaces, which has not been how daemons are isolated in UNIX or by systemd. For example, if you were to ever run OpenCode as root, any local process can trivially gain root as well.