You have a couple misconceptions here, (most) Animes don't run forever till they get cancelled, they write out a story (usually adopted from a manga) make it, then air it, they don't just beat a dead horse until the ratings tank like american animators, so comparing show lengths / reruns isn't fair. As for the lack of anime on american TV today its not because the demand has gone down, its because its gone UP. Anime is now more popular in america than ever and the only reason its not on TV is because they can't compete with internet in supplying anime fans with new shows, many of whom have grown to prefer subtitles to crappy american voice dubs anyway. Why would you wait a year to watch a poorly dubbed anime on TV when you can watch it online a few days after it airs in Japan?
and if you want to bring recent Animes into the mix, I dare you try and compare Ben 10 to shows like Fullmetal Alchemist or Steins Gate