This is true. But they would at least design the app around maximizing user satisfaction with the service (to keep you paying), vs maximizing time spent on the app (i.e. through making it addictive) in order to increase ad revenue. The current incentives are perverse.
That's a nice idea, but then there are all of the times we've started paying for services to have an ad-free experience, only to then have them toss ads back into the mix.