So whose problem is it really then?
You mean via plugins as is currently the case?
My view, is if you are genuinely perplexed I would wager you’re not as actively cognisant or appraised or are empathic to “consequences” because there may be no apparent value in teasing this out, since the horse has already bolted and barn door is swinging wide open so everybody lets go!
You say cancer, more like quantum-weapon of mass [fill in the blank], however some will think that is all hyperbole. That’s cool. Everyone get’s to have an opinion, but I"m thinking overall and not specifically Discourse, what you do with the dev is up to the way that works so to clarify the point in relation to the OP:
The primary reason I am asking is as an admin of Discourse, where you have a duty to a user base, and while this might be a tiny segment in real market terms, but still your user base or portion of your user base are important, and may actually wish not to use a platform that engages any forms of this generation of AI when it comes to the content they generate, for lots of different reasons. The same way they might use adblockers, they don’t want to be part of this “thing” - when that question comes form the first user that asks its because there was no clearly indicated AI policy you have to also have some form of an answer that is truthful.
Therefore segregating the AI tools as plugins that are not core is a good approach, but if it came to it that the AI was core, say in a future update, it might leave room for hesitation or total rejection of the platform.
As long as this line remains clear and delineated then that might be all round satisfactory way forward and clear dev guidance and assurance might be important here too.
I also understand it’s hard to frame it to everyone’s total agreement, but here is the nub of the issue now, this is what happens when a question is asked for a real reason. If if the reason is founded or undaunted it will be the basis for users to make value decisions that convert into action.
If it’s simply a case that a simple statement that there is no ChatGPT or similar AI in the core of Discourse, with a caveat some AI is used in dev and creation as a tool, that may be as close to a satisfactory answer for the ordinary punter on a discourse forum.
Overall the issue of how AI is going to affect the web and by virtue the world has already been done and seen in terms of application and consequences, this phase of releasing these “toys” officially, i.e. giving a pandoras box with API’s n’all to the dev world and again by virtue the world is a ramp up, since before it was a very controlled experimental learning and relatively clandestine phase where this AI had been infused in a big part of the web for years without many ordinary people even realising it.
While I’m trying to keep it within the frame of my question, if you want to really be meta, and discuss the metaphysical then I say go for it, but that topic is already going here isn’t it - How are we all feeling about ChatGPT and other LLMs and how they'll impact forums?
Another aspect are say Ads, I’d assume Google has been using AI of this ilk for ages in its ad serving and even analytics services, but at least for now users can deploy some blocking software and use VPN’s, do those tools use AI too?! 
Maybe it boils down to how people perceive AI and maybe the easiest way to articulate that is some see it as sandboxed (tools), other see it as a fire in the very fabric of the reality matrix that needs to be quenched, either way it’s happening and it’s going to happen whatever it is but there are always consequences, and sometimes all you have is a choice of being part of creating those consequences or mitigating them, what we have with “industry” is they are IMHO being both.