Demand created sidekicks and the lack of demand has ushered in their irrelevancy.

The superheroes of tomorrow won't have sidekicks because of how fundamentally the demographics of comic book readers have changed. Plus I think for the most part the [i]tone[/i] of comic books have changed so much that they would be out of place. We accept the Robins of the world because they have been grandfathered in. Were a character like Batman to spring up tomorrow people would say it had jumped the shark if it introduced a scrappy, young teen sidekick.

That's not to say that legacy characters are done with - just that they'll likely either be adult "sidekicks" like Bucky and/or Falcon or in their late teens. That way you still get some semblance of a paternalistic relationship whilst not stretching the suspension of disbelief too much. (See: John Blake in TDKR)