Don't get me wrong- I am not implying in anyway that you are anti-engagement. You are making a case for the over-importance given to this particular metric.
What I always feel is that there is no one size fits all approach. Sure, for some industries engagement has higher returns and for some, less so. At the end, if it is a copy-paste approach rather than finding a unique voice that sits well with the platform, then it is certainly not conceived with the right intention and frame.
The #lightsout campaign was good for engagement because the baseline objective was to chime in on the conversation.
On the other hand, the #shareacoke campaign had well defined objectives and it not only had positivie brand impact, but also on recall, engagement, advocacy and sales. Were you able to find a can with your name, Gary? I could not, not that straight-forward in my case 😂
The tic-tac in space campaign is another example that comes to mind about a FMCG brand, that was a sucess across engagement, shares and sales charts.
For adventure enthusiasts, the red bull campaign out in California would also be moving. Someone may walk in to buy a Monster can but then the person recalls the Red Bull campaign and end up buying a Red Bull can. The impact would be even higher if the said person witnessed the Red Bull campaign first hand.
Someone may have engaged with the brand for a campaign of this sort but made a purchase week(s) later. There are spillover effects that are hard to attribute.
You mentioned reviews and attributes as more part of the customer journey and not attributed to engagement on social. But there is an inter-link between these two and giving significance to one over the other for sales attribution may not be the best approach the analyzing marketing campaigns. In Marketing surveys, it would be helpful to ask 'How did you get to know about the product/What made you consider the product' and subsequently, 'what was the factor that made you sign on the dotted line' to get a holistic picture. For eg: Someone is in Walmart with their child and the child is adamant about buying snickers because he saw a social media post uploaded by the brand not long ago and engaged with- but the parent makes the decision because of the half-off sale, the sale was due to engagement and demand in the first place.
Also looking at the up and coming platforms- like tik-tok, an MIT research found engagement and product placement together to lift sales. https://medium.com/mit-initiative-on-the-digital-economy/what-makes-tiktok-video-ads-tick-9486ed94724c
On other hand, UGC about a Lambo would not translate.
This is why Marketing is inherently beautiful- all opinions and advice can be equally correct, just depends upon the positioning and how all the pieces fit together 😄