A legacy series is one that is passed down from generation to generation, either in its original form or subsequent re-boots.
There are marquee television franchises that stand the test of time: Star Trek, Twilight Zone, episodic comedies like Cheers, Seinfeld or MASH which run constantly in syndication.
There are museum series which hold great admiration in the industry and general public for their trailblazing formulas, nostalgia or historic significance such as Carol Burnett Show, Laugh-In, The Tonight Show with Johnny Carson, or Your Show of Shows.
There are long form dramatic series which live in the outer memories of television buffs, such as Roots, The Thorn Birds and other kings of the mini-series genre (which was TV adaptions from successful current novels.)
The one thing LOST had from its very beginning was critical acclaim and an immediate core audience. And in the early internet community building aspects of web pages and fan sites, the interaction between the media writers and the fans created the "buzz" which sustained the show in its early seasons (and in some respects caused it to go off the story rails).
As we learned after the show ceased production, LOST was supposed to have been an episodic adventure series. A person unfamiliar with the show could watch an episode without needing a vast pre-existing knowledge of the character or story lines. The formula would have followed Star Trek, which can be viewed in any specific order since there is no tied plot lines from season to season.
If LOST had followed that episodic path, it would have pushed its way into the conversation of being a legacy television show. It would have been able to have a real independent syndicate run (even though hour long shows rarely do well in syndication). It could have found a secondary or new audience in that wide syndication if new viewers could be immersed in a resolved 60 minute story. Even shows like Breaking Bad or the Sopranos had contained episodes which may be leading to an ultimate conclusion, but LOST broke the mold and allowed story lines to skip weeks, get hidden by new mysteries, and overwritten by conflicting back stories. Part of the problem with a hour being divided into island time, back story features and then alternative universe sideways stories there was no true resolution of the multilayered plots after each episode.
LOST lost itself in its methods of story presentation. Die hard fans could sentence diagram the various plot lines and character studies during the week between episodes, creating vast theories and predictions which were as entertaining as the show itself. But a casual viewer's confusion would be an instant turnoff. There were many instances where original fans tried to rope in their friends or spouses into watching LOST after the first season, and the push back was fairly strong after only a few viewings. It was harder for someone to pick up the series midstream than an original viewer.
It probably did not help that even die hard fans could not explain what was going on with any certainty. Some viewers merely want to sit and have the screen "entertain" them without any active participation. So as breakthrough as LOST was in challenging viewers to see the mysteries and find clues to the questions, it did a poor job in holding the less attentive viewer. The ratings showed a slow decline from Season 2 to the end.
Then the deep split in how The End was viewed by critics and fans sealed LOST's fate of not being a legacy show. For those who enjoyed the finale, LOST will remain a coveted memory. For those who hated the finale, there may remain some bitterness and frustration to "move on" away from holding the series to their hearts.
Could LOST have been more episodic like other science fiction shows? Yes, but that would have taken crisper and more focused writing. Could LOST have tied contained episodes together to weave subtextual story lines for the hard core fans? Yes, but that would have taken deeper planning and clearer execution.
It seems that once LOST was green lit, and rushed into production, it took on a life of its own; an uncontrollable beast much like the smoke monster.