From what I heard, there are other objects they have found that are bigger than Pluto. How many "planets" do we have?
Quaor and Sedena are roughly the same size as Pluto, and there could be hundreds, maybe thousands of similar size objects out in the Kupier Belt and Oort Cloud, some bigger, some smaller. Some of these newly discovered Plutonians have moons like Pluto does too. Besides the other criteria for "planet-ness" of orbit circularity, clearing its orbit of competing bodies, being in mostly the same plane of the ecliptic, it was the the probability of many icy dwarves which did the most to demote Pluto.
Because if Pluto is a "planet" then our Solar System could conceivably have a thousand "planets".
Then there is the issue that there is likely a "planet 9" out there, anywhere from Earth-sized to Neptune-sized, based on the gravitational disturbance of the Plutonians and comets we've detected.