Even if LLMs didn't advance at all from this point onward, there's still loads of productive work that could be optimized / fully automated by them, at no worse output quality than the low-skilled humans we're currently throwing at that work.

Exactly, the current spend on LLMs is based on extremely high expectations and the vendors operating at a loss. It’s very reasonable to assume that those expectations will not be met, and spending will slow down as well.

Nvidia’s valuation is based on the current trend continuing and even increasing, which I consider unlikely in the long term.

They already are paying off. The nature of LLMs means that they will require expensive, fast hardware that's a large capex.