The compelling argument for continuing to call Pluto a planet is that it's large enough to be self-gravitating. There's no doubt in the mind of any serious astronomer today that Pluto and Charon are Kuiper Belt objects. But that is not a bar to the designation planet. When the term 'minor planet' came into use the distinction adopted at the time was that minor planets were not large enough to impose sufficient self-gravitation to make themselves spherical. Pluto and Charon both obviously do have that measure of self-gravity, as does this newly discovered object. Historically the large moons like Charon and Jupiter's Gallilean moons have been designated 'moons' because they are gravitationally bound to their companion planet, so Charon, the Gallilean moons, Titan, and our own moon have been excluded from the count of planets even though they would otherwise qualify. They are all certainly too massive to be minor planets.