I don't think tube sites affect sales in the way that people think... Hell, there's a reason why tube traffic upselling to a paysite converts... The user wants to see more of that porn.
Tube sites are great if you want a ton of generic "porn". As soon as you're looking for something in particular, they become a pain in the ass. People will still pay for quality and they will pay for what they like (I personally can't stand watching porn on a tube site, the video quality just isn't there)...
Especially when you drill down into niches, that content is even harder to find on tube sites... Most of the more obscure fetishes and microniches have only seen their sales get stronger. Obviously if you're promoting hardcore teen porn and only upselling bangbros you might see a decline in sales, but that would happen with or without the tube sites.
Personally before I started doing this for a living when I surfed for porn I could get off from a 1 minute hardcore trailer on a paysite tour
Having longer videos available for free won't separate the users who want to pay and the users who don't want to pay any more than they are now.
Shrug.