You used the word anthropomorphize twice so I am guessing you don't like building systems whose entire premise rest on anthropomorphization. Sounds like a reasonable gut reaction to me.
I think another way to think of all of this is: LLM's are just pattern matchers and completers. What the training does is just to slowly etch a pattern into the LLM that it will then complete when it later sees it in the wild. The pattern can be anything.
If you have a pattern matcher and completer and you want it to perform the role of configurable chatbot. What kind of patterns would choose for this? My guess is that the whole system/assistant paradigm was chosen because it is extraordinarily easy to understand for humans. The LLM doesn't care what the pattern is, it will complete whatever pattern you give it.
> And part of what I find troubling is how casually people (prompters and users) are willing to go along with the ‘you are a chatbot’ fiction.
That is precisely why it was chosen :)