Here are some random reasons why internet porn doesn't seem to be selling as much as it used to:
1) When internet porn started, that's when content creation for the internet started too, which meant porn was new to everyone that saw it at the start and took a while for things to get oversaturated and higher quality to make the older porn seem worthless to buy.
2) Much more webmasters are promoting the same thing since more people have computers and are in need of making a few extra bucks these days and hosting is super cheap compared to the 90s (Affiliate marketing got me out of poverty)
3) Blackhat and cookie stuffing cheaters
4) Some 'webmasters' just suck and like to complain about it even if they've never made money in the past either..

5) Internet connection speed is much better than before so promoting only image FHGs doesn't quite cut it anymore by itself..
6) There's a lot more to do on the internet nowaday than just research and jerk off like it used to be...
The point is if you have good content and promote it well with good selling text, minimal leaks on the page, frequent members area updates, great landing page with clear info of what the surfer is getting, and know how to promote your niche, then you can make good money. The amount of money that people spend on porn is determined by how good you are at selling.. ;)
I don't think tubes are the problem..