Why the sudden pivot to agents and subagents? One shotting not good enough? Was a tech plateau reached? Or maybe one related to cost?

LLMs can be a massively transformative and valuable technology, and this would still be a bubble, there's no reason those two things can't be true at once.

In fact "pure" bubbles where the focus item is of literally no value (tulips, NFTs) are quite rare. Much more common are the bubbles based on an actual real transformative innovation (canals, railways, radio, internet, LLMs).

Railways did absolutely transform how travel worked in the UK, while simultaneously almost everyone who invested in them lost their shirts

That's also what NFT hypebros said.