While like I previously said I think ET stands up very well on it's own nostalgia aside I don't think the nostalgia factor should be dismissed. Truly excellent attractions much like movies or books over time become classics. Nostalgia and in turn the rides status as a classic is a very valid reason not to replace it. Look at things like It's a Small World and Peter Pan, these are some of Disney's most popular rides. Even with large throughput they can still be some of the longest waits in the park. Those rides if built today would be a disastrous joke of an attraction. I think ET falls somewhere in the middle here. From an attraction point of view it's far better than older rides like many of the Disney classics, this is then combined with the nostalgia factor.