Brazzers and others are, on one hand selling porn via sites teaching surfers not to pay for porn.
It's like MacDonalds giving hamburgers away from a street vendors trolley advertising hamburgers. Works today for a few because they are soaking up traffic in vast numbers.
Tomorrow is the question. In the future, will teaching surfers not to pay for porn leave them 50% or even up to 100% reliant on ad sales for profits. Selling penis pills that don't work, dating sites with fake girls and Cam sites? Which surfers might start to realise is not good value. Or in danger of being taken over by other sites giving it away or slashing the prices.
Cutting the number of free full length scenes won't work, surfers who thinks porn is free will move to the next Tube site giving out full length scenes. Making content that's so fucking good it has to be bought might work. Until other Tube sites realise stealing it is easier than making their own.
Maybe advertising will support it long term, but with such a limited base of advertisers willing to promote via porn sites it's a risky business.