While a fad (in this context) depends upon a company maintaining a product, the act of maintaining a product is not a measure of how long the fad lasts. Take Facebook, the product. I'm fairly certain that it is long past its peak as a communications tool between family, friends, and colleagues. Facebook, the company, remains relevant for other reasons.

As for ChatGPT, I'm sure time will prove it is a fad. That doesn't mean that LLMs are a fad (though it is too early to tell).