Well, there are seven other known sub-planetary bodies (dwarf planets) in the solar system that also have moons: Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia. Eris in particular is larger than Pluto. So then Eris would have to be promoted to being a planet, too. Along with the other six, plus any more we might discover later. At the time of its discovery I don’t think anyone had yet observed that Pluto has a moon, and it wasn’t discovered until even later that Pluto actually has five moons.