Does the additional ad spend convert to an acceptable lift in sales? If yes, why do you care about video engagement? If no, why maintain such spend even if the engagement numbers are genuinely high?
Most small advertisers wouldn't be able to do this, and attribution is hairy as hell, but the options for measuring this are there.
So, we make decisions based on imperfect information. When it was TV or Radio ads, there were 3rd parties that measured the market and reach of the station. With Facebook, Youtube, etc we have to take their word for it.
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?