The opportunity cost of it though, like every major feature decision in a browser, is presumably very significant. What could they be doing that their users actually want?

As a dyed in the wool Firefox user, I am of course very used to disappointment about how they direct their development effort. There are a million things I'd rather see them work on than strapping in an LLM. I don't know much about the Vivaldi community but I assume they took this into account when making this decision.