You don't measure the conversion in case of a brick and mortar vendor. But you can still measure proxy values for effectiveness: an uptick in sales volume or number of clients after after starting a campaign, for example.
Also, our marketing doesn't exist in a vacuum. Weather, competitors, a traffic jam down the street, variations in employee performance, etc all impact the performance as well.
It's not like we can just A/B test Facebook video ads vs Pandora Ads and pick the winner.
So, we shift our budget based on the available information.
And the average video view length is part of the information that we used to make decisions.
I know multi-touch is hard, but doesn't that mean you should be less confident in your statements because the data is fuzzy?