Did they ever base an episode around Britney or NSYNC? Nope
Plus, Britney is still around and relevant in pop culture and NSYCN gave birth to Justin Timberlake, who is huge.
I seriously doubt Miley Cyrus is going to be a big name in 5 years, maybe she will if she has a mental breakdown similar to Spears or Lohan, or changes her act, but I just can't see her having the staying power especially because she is so closely associated with the Disney brand, granted even if she did leave Disney, people would still see her the same way unless she starts doing cocaine and hanging out with Paris Hiltion.