I've always believed that porn is addictive, not like heroin, and the more you see the more you want.
So if someone gets hooked to looking at porn on free sites would it follow that once hooked many will eventually end up signing to a paysite. Not every one but a fair proportion.
I believe there are people who will never pay for porn, they will go without it. People who can take it or leave it as the mood arrises. Then the guy who will spend hours looking at it and collecting it. You cannot base an industry on the first two types, it's the least one we earn our living from.
He will click on TGP after TGP and like a fish will nibble and move on, then he will see something he cannot resist and click on the link. Depending where he ends up will depend on whether he moves on or takes a big bite. Now you have him hooked.
Free sites would die the moment people stopped clicking on that link and buying something. What Paysite owner would pay out $1,000 for banners and placements on them. Then court the webmaster, spend $1,000 advertising to them, spend even more at shows to entice them, give them free galleries to post up and then offer them huge percentages to basically post to free sites?
Don't fear TGPs they may create more buyers than they steal.