They don't think so. And they have the internal numbers to back up their decision. HBO is a very smart and well run company, and they make buckets and buckets of money from their deals with the cable companies.
You (and I) would love to be able to buy access to their content without paying Comcast (or DirecTV &c.) but the number of people holding that position is VASTLY OUTNUMBERED by the number of people willing and able to pay $40/mo or whatever on top of what is considered a utility service. That's not going to change fast, and there's no point for HBO to get those arrows in their back when Apple/Amazon/Netflix are charging ahead regardless.
Just as a point of comparison, Netflix, with a much larger catalog of shows and movies, has something of the order of 25M subscribers
Obviously an overnight change would not happen, there would be a transition over several years and the streaming subscribers may well be supplementary to existing subscribers. But during the transition, cable company partnerships would be strained and existing revenue would be at risk. Not to mention the capital investment required to provide the content reliably, dealing with the multitude of streaming devices, providing customer support, marketing the product directly to consumers and all the other things that cable companies take care of (albeit badly). Netflix is making bold changes in its business model and it's certainly not an easy thing to do.
Ultimately, HBO's core competency is in creating amazing content. It's arguable if they'd make more money by changing their business model, but I'd wager that HBO execs simply do not want to have to deal with all the distractions from their core mission that would occur by doing so.
The only way they could lose revenue is if current subscribers dropped cable and didn't go with streaming, which seems unlikely to me.
Cable companies are already losing subscribers. They will do everything they can to discourage people from defecting to broadband downloads. One of the most obvious things they can do is to have exclusive content. Which means they would be smart to put immense pressure on HBO to stay out of the distribution business.
They've done the math, and they wouldn't make more money than they do now. That's why they haven't done it yet. It's not just pure subscription numbers that matter -- it's the millions in free promotion that cable/satellite companies provide that must also be factored into the cost equation, as well as the millions in costs they would incur by expanding their streaming options.
I doubt that. Rather, I bet they've made a conscious choice to only be a content provider, not a content distributor. That's a fundamental business change. I say this because if HBO decided to make the change, and enabled, say, a $10 online, streaming subscription, I think they'd make more money than they do now. But that would likely upset their current deals with the cable companies, and it would be a fundamental business model change.