The question of whether web browsers will one day be expected to normally include an LLM is not at all relevant to the situation at hand, where that absolutely is not the expected or typical behavior of a browser. Google should be asking for consent now even if they expect in several years time to be able to presume consent for this.
So the principle is that any as of yet non-standard addition to software needs explicit consent from the user?
Major feature updates require consent, yes. Automatic updates are for bug fixes and security patches. It is disrespectful to the user to automatically deploy any update that changes the UI without warning or consent. I like my toolbar buttons to stay where I left them.