I think TNO is the worst form of compromise. It is a compromise simply to be one. It's not a good category of 'space thing.'

As we learn more about other star systems, we will certainly find more planets. There are almost certainly other moons. There are probably comets and asteroids, and something like a Kuiper Belt and Oort Cloud around most other systems.

Nowhere else in the universe will there be other trans-Neptunian objects, because nowhere else is a planet named Neptune. Even if we change it to 'planetoids that have orbits partly inside the outermost planet,' what's the point? It's a pretty unimportant property to make a whole category out of it.

The distinction between planet and planetoid is gray - in nature, not just in astronomers' arguments. I have no problem with the decision to keep it as planet, since it was arbitrarily designated as such originally.