> these are ALL diligently measured at every stage
Mmm. Sometimes. Sure, you can obsessively measure audience and sales numbers, but if you don't have any controls then it becomes nearly impossible to figure out how much of that came from ads and how much came from everything else in the entire world.
If Coca Cola cut advertising 90% this afternoon, they likely wouldn't see the consequences until a new generation or two grew up.
There's also a huge mutual back-scatching thing going on. I remember in a former life we wanted to pass on a big software vendor's user group show because, while we sort of needed their software for some important customers, we got very little traffic at this expensive event. Their CEO called our CEO and basically said to him "Be a pity if something happened to our partnership."
You seem to be assuming that the potential gains are limited to current players of the game in this comparison.
It's possible that Coke, Pepsi etc. are all losing more money on ads than they're gaining in market share against their current competitors and in increased sales. But that doesn't begin to address the question of whether advertising is a net loss.
For one, the advertising also serves as a collective moat against new competitors, which could enter the market and eat up both Coke and Pepsi's market share. Maybe their biggest gain from it is that a bankrupt soda company from the 80s didn't replace them, and that a similar company today wouldn't have room to enter the market.
It also doesn't account for more complex effects like a net increase in sales for all companies in that market. E.g. maybe Rolex, TAG Heuer etc. all benefit in the long term from expensive watches being seen as fashion accessories.
If Rolex stopped their advertising spend one month and saw little immediate change, then TAG Heuer etc. might follow suit. But both might only experience the real loss years later as "luxury watches = must buy" faded from the psyche of their current and potential customers.
Also, there's one alternative that's often forgotten in these discussions: Perhaps game theory is at play. It may be, for example, that, across entire industries, advertising costs more money than it's worth. But that everyone has to do it anyway, because anyone who chooses not to will start losing ground to everyone else. IOW, just like in the standard prisoner's dilemma, choosing to act is less about increasing your potential gains than it is about limiting your potential losses.
There is an interesting long-running natural experiment in the pharmaceutical industry that suggests, albeit inconclusively, that this is the case.