Final elimination of all originality
"the agent could detect an issue that calls for creation or revision of a graphic, synthesizes internal chat about it, considers the content of a company's brand guidelines [...] and squirts info over an API to SaaS-y graphics service Canva, which would use generative AI to produce a candidate image"
As none of these agents can come up with anything other than based on what already exists in statistically significant quantities, the results will inevitably be bland and lacking in originality. The most effective adverts (the ones that stick in the cultural mind) did so because they were different from the mainstream.
However this is not a new problem -- many decades back I worked in print production and found it extremely hard to get clients to understand that starting their brochure design from samples of competitors'' literature was unlikely to result in theirs standing out among the mass. Technologies such as the proposed have probably been conceived primarily because that message has still not sunk in. But it will of course allow the graphics team to be dismissed, so the bean counters could be happy (unless of course the cost of the tech turns out to be comparable).
Oh, and BTW, I'd hardly class graphic design as 'office drudgery' -- even commercial art is a skilled trade.