If HBO were to make it a standalone subscription option on AppleTV, that would be a "killer feature" for a segment of the market.
I can see that coming sooner or later but probably premium priced for the cable-cutter benefit. In other words, one might currently think about the HBO bundle at around $10-$15/month and some will then throw out that they would pay about $25/month for HBO GO alone (if they didn't have to have a cable subscription too). But I imagine the actual price would be something like $40-$50/month for HBO GO alone, looking at the price of premium adult channels as a bit of a pricing guide.
So many of us dream of al-a-carte but don't want to pay what the dream would likely cost. We imagine it's something like 50 cents per channel but it wouldn't be. Instead it would involve pricing to keep the existing players making at least what they are making now PLUS let Apple squeeze into the chain and take an additional amount. And if we dream of commercial-free, then we are killing the other people's money stream that helps make television cost what it does now, meaning if our dream is of commercial-free al-a-carte, we have to make up for the loss of that subsidy too (and it's not even close to what anyone would call "cheap" in and of itself).
Al-a-carte seems like a wonderful goal where we somehow pay a lot less than we pay now, Apple gets to squeeze in and make a big cut and everything like quality and breadth & depth of new programming just magically continues to flow. But if there is ever a solution where the consumer masses pay a lot less than they pay now, somebody else in the chain takes a big hit... some quality of programming and/or new show creation and/or variety of programming is likely to plunge. That's great if the crowd that votes for the survivors happens to align with one's own tastes... not so great if your favorite programming is not mainstream appealing.