Sky will be a different business for sure but it is a dominant player, is extremely profitable and has the money to develop the online delivery. It will face more competition but with 830,000 subscribers that is a huge behavioural shift your are talking about. And there is not one single service out there that could replace the combination of sport and entertainment that you get with sky
Younger generations are extremely comfortable with managing multiple providers for their entertainment needs. Not much different to having spotify, or an extra utility bill. They will never subscribe to Sky given the high cost - these generations are used to paying for exactly what they want, not paying large fees for entire genres of material they're not interested in.
People in their 30s/40s are the people we're seeing weigh up Sky versus going 100% online. Some are staying, some are going, so the overall subscriber numbers in that demographic are coming down.
People in their 50s and older are most likely to stick with it long term, but numbers won't be increasing, just a slow decrease as they die.
I see no demographics where Sky subscribers could possibly increase, unless as you say, they develop online delivery.
In terms of that I think you're missing out their main competition in your comment - overseas providers with FAR more money. Netflix is massive, Panasonic are starting to release TVs with a Netflix button. Sky have got no show against outfits like that. They simply don't have access to the material people want to see, at any price. Their attempted introduction of online offerings has been farcical, and the quotes coming from their top brass give me no confidence that they're attempting to move with the times at all.
They have virtually no power in the football rights business at the moment - even someone with serious performance issues and little infrastructure like PLP easily outbid them, just to trial it out with a view to the asian market.
Sky have lost the last two bids now, and will only get to show it next season if BeIN decide to recoup a little cash - that's probably what will happen but it's not a given. Sky's money is smallfry to someone like BeIN, they could try another trial situation like PLP if they're in that mood. They do have Champions League too so could easily try out some kind of online football channel here as a research exercise.
Whatever does happen, Sky won't have much influence on it.