I think you're misunderstanding the severity of the lethal trifecta. Just because you put access controls around the LLM doesn't mean all that much if the access controls allow anything in & out. There is no way to write a shim that blocks "everything naughty", while remaining useful.
You literally have to fully prevent all outside input, or you have to prevent all exfiltration routes including web page reading (even the choice of links to follow is an exfiltration mechanism). At that point, what's left? What do you think will be on your allowlist?
I seriously doubt the early adopters of these software bundles use their assistants like with such restraint (https://xcancel.com/summeryue0/status/2025774069124399363), and that idealized image of these access control shims is not realistic.