The problem is not HBO, the problem is Comcast. If HBO offered all their premium content a la carte over the internet for competitive prices, HBO would be fine. They would make even more money than they do now -- except that the cable companies would collectively go berserk and retaliate, the fear of which is what stops them.
If the DoJ had any interest in not looking like a collection of apathetic former RIAA lawyers there would be an antitrust probe into this sort of thing by now.
No. People would ridicule them.
The primary problem is that HBO would have to transition their subscribers to their internet service all at once or they would lose subscriptions. There are still plenty of HBO subscribers watching on analog TVs with no internet attached to them, and HBO can't take steps to make itself independent from the cable companies without causing them to retaliate by depriving it of all that subscription revenue.
People pay to subscribe to HBO (tv). If people could get a lot of that content by subscribing online, for cheaper, then lots of people will switch over. Which means HBO disrupts it's own cashcow moneymaker with something that makes less money.