Enforce too much ads and people will go elsewhere, but now we have the adblockers so people will go wherever they like. That's the reason why people no longer watch TV - they can't avoid the ads that spams them with perceived higher audio volume, stroboscopic effects causing epilepsy and doubling the time a show goes on.
Ever considered why some sports are more available on TV than others? Baseball and American Football - you can inject ads often. The long full course yellows on motorsports are great for ads - and they are a lot longer and more frequent today than a few years ago.
Also look at magazines that have died over time. A magazine that was once popular and thin grew fatter and fatter with ads while a lot of the content became watered down and suddenly people stopped buying that magazine.
That said - I can live with ads if they aren't impacting on the stuff I want to access. Often a text ad of around 10 words is enough to catch the attention of anyone that's interested. Throwing up a full page ad covering the content alienates the visitor completely and will result in people looking for a belt-fed shotgun to clean out the company behind the ad.