Sure there could very well be hundreds of millions of people world wide who have a device they can use to buy/read digital comics... But how many of them are comic book collectors, teenagers & young kids who may be interested in reading comics?
Digital is a great way to make comics accesable to people in locations without comic retaliers, but as a way to reach the average man on the street its dreadful.
The idea is to make comics more accesable, not less.
The reason pirated comics are so popular is that you can get literally everything that came out in a given week at the price of "free." Not only is it cost free, its also DRM free. Digital comics, the only medium in the world where the bootleg is better then the legitimaste version.
As for marvel cancelling series, the problem isn't "that most didn't pay for them," the problem is that with digital piracy, marvel gave them no reason to pay for them. Thats the thing with the interent: We can now check the quality of a book before we buy. If the quality is just not there, then the book doesn't get sold. An lets be honest, MArvel is well known for low quality books, especially mini-series.