Sure they can't. But the "frozen" thing allows for a more expanded period of time between the disappearance of the characters and their return. I mean, they can put Jay in the limbo in the 1950s and have him back in the 1980s. Or the 1990s. or the 2000s. Or, in the case of Alan Scott, they can have him stay a bit younger because of internalized powers or something like that. For the most part, they are all non-issues.
I kinda agree that the situation is a bit more problematic for some of the members of the cast (villains, wives, and so on), but I really cannot see this but an opportunity to create stories. In fact, that's exactly what they did in series such as JSA or Starman or Hourman - which were rather successful, or critically acclaimed, or both for a very long time. In those series, some of the old characters died. Some of them retired. Some of them ended in dimension X, some of them changed, and some of them had nephews.
The point is... Theoretically speaking, most of the objections to the concept of Legacy may have some good points. But when you take into consideration how the stories were tangibly written, it's very difficult to argue that, for the most part, the writers were very skillful at circumventing all of the obstacles and using the idea to build something interesting and well done which COULDN'T be done with classic major characters: creating stories which were focused on the concept of unforgiving, relentless time, or family. That's a pretty great achievement - and it is one the reasons for the JSA series to last so long.