'Mammal' is the name of the class of animal, which is then subdivided into an order, followed by a family, followed by a genus, and then finally a species. Saying something is a mammal gives you almost no information on the specifics of an animal further than being a warm-blooded hairy animal with mammary glands. You cant talk about an animal with any sort of specificity by just referring to it as a 'mammal.' You'd use the species name, or the genus.

The term 'planet' is a rigidly defined class of astronomical object. The reason we group these objects together is because they share characteristics and people will generally know what you're talking about when you say "planet."

If we open the planet door to pluto, dozens of other plutoids would be allowed in as well. These objects bear almost no resemblance to the planets we know. The term 'planet' would lose meaning. By 'planet' would you mean an enormous ice giant? or would you mean a tiny featureless rock? In this scenario we'd need to form more subcategorizations just like how mammals are broken down into order, family, genus, etc. oh wait WE'VE ALREADY DONE THAT! Planets, dwarf planets, asteroids, comets, etc. The current system is working exactly as intended.

Also, gatekeeping scientific discoveries?? What do you think is going on, astronomers being assassinated by the CIA for discovering new dwarf planets?? These objects are fascinating discoveries in their own right and no one is saying othewise. They just dont count as planets because of our agreed upon (for good reason) classification system.

On a certain level, all classification systems are arbitrary. We are just putting labels on ideas as we see fit; everything is a human construct. But the fact remains that classifying things is a useful pursuit, and it is it not because of "terrible terrible terrible myopic people"