Do you think maybe FTL got kind of “mixed in” to Storybrooke when everything swirled together as the curse happened? Maybe that explains what Mr. Gold was doing when he was doing “a spot of gardening” in the forest. Maybe he was digging up FTL stuff that was mixed into the dirt in Storybrooke as the curse occured to bring back into his pawn shop. So maybe FTL is necessarily underneath Storybrooke, but just like the characters, is kind of a part of it? So yes, it’s destroyed in terms of the way it used to be but remnants of it still exist and might still be found throughout the series – maybe things that will be found will help to spark people’s memories?
Personally, I like the idea of the characters remembering who they are but choosing to remain as their “modern” selves reunited with their FTL significant others. That way the series can last longer!!