My problem with any ad model is it incentivises the maximisation of views/engagement that allow for more/stickier ad impressions which means a proliferation of viral/tabloid/divisive content like youtube/snap/facebook. Once that incentive can be minimised, the incentive to provide quality product that users then feel willing to pay for can become more important.
Of course, these networks didn't attract the user numbers they have by charging up front so for any challenger to come along and try this model will be extremely difficult due to the aforementioned friction in either getting out your credit card to pay or paying up front for an app. People will gladly shell out hundreds of dollars for a new phone but won't shell out a fraction of that for a quality app on that phone.