> But no serious linguist thinks that kiki-bouba is that important to language.

Do you have a source on that? Because I would expect anyone studying sound symbolism to find the bouba-kiki effect extremely important which is probably why it's such a widely cited study, also inside linguistics.

It's hard to find a source for that kind of negative statement.

Kiki-bouba is important for sound-symbolism definitely! But sound-symbolism is marginal when it comes to language. Iconicity and similar things are very interesting phenomena but they're not the difficult part of language at all and they're not necessary parts of language.