Any Kuiper belt object big enough to be round is a planet!
OK, how many planets are there in the Solar System?
Err, we don't know. We have a very vague guess at lots and lots.
OK, how many planets do we know about?
Err, we're not sure. Some of them are just blobs of light. We don't know how big they are. Or how massive they are. So we don't know whether they are big enough to be round. And anyway some of them might look big but actually be two or three separate small bodies orbiting each other.
OK, what sequence are they in, counting outwards from the Sun?
Err, we have a rough idea, but we're not sure of some of the orbits, and the orbits are so angled that the distances overlap.
But we do have a list of definite possibles we can put in school textbooks?
Err, no, because the list keeps changing as we get better observations.
We need a word to describe the eight inner planets - the four rocky planets and the four gas giants. The things we usually think of as planets. Now we could invent a new word for those, and use 'Planets' for those plus Ceres plus the Kuiper belt plus anything else we haven't found yet. But that would mean persuading the public to use that new word. Ain't going to happen.
So call those eight 'planets', and use a new term for all the rest. Anything else just isn't going to work.
So do we include Pluto? Even though there are almost certainly many K-belt objects that are larger and more massive?
It just doesn't make sense. It needs a much better justification than some US States made a law about it, or lots of people saying "I want it to be a planet, and I'm going to hold my breath until you change your mind, so there!"
It seems to be pure sentimentality. So are schoolchildren in a few hundred years time going to ask "Why is Pluto a planet when all the other similar objects in this Solar System and other Solar Systems are not?" and be told "Well some people in the 21st Century screamed about it, so we messed up the whole classification system just to shut them up."?