And a great way to give permission to read and modify all websites you visit to something that should really better be an isolated application.
I would really not consider web extensions an alternative to standalone apps. In my security model, they inhabit the opposite corner.
> some users like to use VPNs securely tied in from the browser itself rather than running the full-scale app
This might be a mistake, for example. A VPN app runs sandboxed on mobile OSes (and many VPN types are supported by OSes natively so there's no need for an app in the first place), so it can't get access to website data (if HTTPS is used), whereas a web extension usually can.