In that case the Instagram ads may be delivering some value!

What you seem to be saying is that Instagram is a good brand building platform over the longer-term vs platform to generate leads and transactions over the short term. Unless, there is a study to prove that I am not too sure. I usually find media agencies and platforms make this argument when they cannot justify the RoI of their platform in a direct and measurable sense. It is much like how Google tries to prove effectiveness of its display ads using a fuzzy metric called view-through impressions.

On the coke bit, coke actually does measure the RoI on its TV using a mix of brand track studies and marketing mix modelling. I know this because I have worked with agencies that Coca-Cola engages to do both and is really scientific and serious about it.