Little game of intra-solar-system pool with Nibiru as the 8-ball?
Not Nibiru (although that would be an amusing name for any newly-discovered outer planets, should that happen), but something created the “Kuiper cliff”, just as something (else) produced these “missing asteroids”. In the latter case, the asteroids may be missing because they’re not really missing at all — this could point to the non-reality of the commonly accepted models of solar system origin.