I’m surprised that some combination of Canvas, SVG, CSS and JavaScript hasn’t replaced Flash. This is established tech and fairly simple. This combo also makes responsiveness easier, IMO.
Flash animation had its heyday when it was the only way to deliver video and games on the web realistically. With video streaming being so easy and having the added benefit of platforms with lots of eyes already, animation is a hard sell.
As you say, responsiveness is a huge advantage for animation, but unfortunately I don't think that is enough to get people on-board.