The problem with bringing back too much old talent is balancing it with todays talent.
People like Cena and Orton are safe, but for every Rock, Kevin Nash or Trish Stratus that gets brought back, a whole bunch of guys like Ted DiBiase, Drew McIntyre and John Morrison get forgotten about. And any fans that no longer watch anymore, probably chose to stop watching because the product wasn't to their liking. I doubt someone will get back into WWE just because Mick Foley is back, or something.
I really like WWE's roster at the moment, so I don't see a need to bring back old stars.
Except Chris Jericho, of course.