Try out some of the example applications at http://degrafa.org/samples/ and try to guess how long it would take you to write those samples using modern javascript frameworks vs how long it would take to write using Flex/Flash.
Flash has always served a niche for what HTML and JavaScript can't easily do; the problem is that over time this is diminishing, and it's looking like over the long term Canvas and SVG will be one more nail in the coffin.