Plans change. I remember some people who talked with D3 devs after the release of ROS saying that the plan for the next expansion is to have the player choose between playing as good and evil. They might have realised introducing such a system might be a better plan for an entirely new game, which builds on that aspect from the start (perhaps even in the skill tree), rather than ro stitch it onto the game in the last act.
Regarding the story, I don't think they've had much regard for that. It is always gameplay first with Blizzard. D2 was an almost complete game before it even got a story, though that story ended up being great. D3's story is arguably not, and I don't think "we haven't finished the story" is a strong enough argument to push out a new expansion if they have no new systems and gameplay planned. So if they feel like the content they introduced in the patches is enough and if the system of good vs evil feels like a good enough reason for a new game, they will make a D4 instead. Either way, we will know for sure by Blizzcon. The limited content in recent patches can indicate both: either they are winding down D3 development since they got all they wanted in there and will now just balance, or they are investing their effort into the expansion and don't see a point in investing in ROS this late in its lifespan. We will know once the Blizcon schedule is released. If Diablo has a lot of time dedicated, we are getting an expac, huge patch or D4.