is that any thief luckier than moll?

Defoe allows Moll to escape because it fits into to the message of his story. You have to remember, that during the time period, Moll Flander's life may have been a little exagerated, but then again very possible. Today, we might very well see her on an afternoon talk show (thinking Maury or Springer here). On the other hand, Defoe also portrays Moll as a mother; it;s part of the story, and yet suddenly the children are no longer a part of the story. If you try to string together Moll's escapes, this would be another, but the escaping fits in with Defoe's reasoning. He writes a "fluff" novel here, but it isn't really "fluff," because the theme is far removed from the actual writing. His writing is flawless and entertaining, even if his plot has holes in it.