I have a friend whose dad finished his engineering career at SABRE (he retired some years before it shut down). According to him, the original concept wasn't far-fetched, but that they didn't crack some manufacturing / materials issues. He thought they should have continued to work on those, instead of redesigning to avoid them. He had retired by the time we spoke about it, and I don't know the particular model / design to which he was referring. If that's accurate, then possibly materials technology / manufacturing techniques have matured to the point that the "simpler" design he preferred could be feasible. That's all second-hand speculation, so take it with a dose of salt, but a short timeline might be reasonable.