Eventually, HTML will grow into a proper windowing framework like a desktop operating system, or something else will come along to do that job. At that point, it will finally be as functional as desktops of the early 90s. On one hand, that's an exciting time to look forward to. On the other hand, that level of stagnation and limitation is incredibly depressing. Websites will always look older and work slower than a local application.
Mokalus of Borg
PS - They'll never go away, of course.
PPS - They just offer too many advantages of centralisation for that.