The thing about the WG erasing people/places from history is that everyone eventually just stop talking about them. It's kinda similar with the real world: can you name every president/prime minister/king your country ever had? Or all of the most relevant individuals in the world from 40 years ago? Even if they're in the history books, most people don't have a clue about them nowadays (especially in the "snack" info era we're in now).
For the One Piece's world it's even worse: as most places are isolated by dangerous seas full of pirates and hazards, most of the info people get are from newspapers (up-to-date events, but still kinda suspicious, since Morgans apparently monopolizes the segment), unverified books (e.g.: Brag Men, most people think that they're just tall stories) or even oral tradition (e.g.: Nika). Places like Little Garden, located at the beginning of the Grand Line, are still uncharted to most countries. Devil Fruits are considered folklore in the East Blue. And so on.
Even if someone is massively relevant as Xebec, it's still easy to forget about them in a generation or two if nobody talks about them anymore (Xebec's a blast, but he's just a menacing wanted poster for most people).