Did they ever say that it was a formal rule for every pokemon to be based off an animal? Did they ever say that they can't make mons out of random items? I think it's safe to say it's a no for both questions, because inanimate object mons have literally existed since the begining of the franchise, and the start of the series had things like a rock with arms, a pile of sludge with a face, and so on.
While there are some inanimate object mons that I'm not fond of because they are just simply bland objects that battle and not much else, that is a very small amount and practically almost every inanimate object mon in the series has some interesting gimmick tied to it's basis.
So why exactly do people complain about these types of pokemon? So far, I have found only one legit reason to this, that being how it just doesn't make sense to find random living objects in the outdoors. And that reason is easily debunked by how this is the same universe where you can fit a gazillion items and even a whole bike into a bag, small birds or even flightless birds can carry grown people for flight, you can catch actual deities into balls, and so on so forth. Tl;dr, the pokemon world is supposedly a fantastical universe, so if the stuff I mentioned above is normal, then some living objects with faces randomly appearing in the grass should also be fine.
I remember an old post across another site by someone with an outlandish username, I think sounded like 'leaf beret' or something weird like that. This was a super old, possibly a decade old post and I think it was him ranting about how inanimate object mons were… useless in battle. Not even regarding design, but I did then check some of that trol's other posts, and could tell that he really was against inanimate objects just for being non animals, which is completely ridiculous.
Seriously, end this nonsense. They never said it was a rule that everything had to be a living thing and sometimes I feel like we should get a generation where half of the new guys are inanimate objects with faces just to see all the salty reactions. But seriously… I suggest this because no matter what, there's tons of cool ideas and concepts that can be made from objects, so no matter what, there's probably gonna be many inanimate object pokemon in future generations.
Edit: If anyone says another "we like what we like and can voice it" comment then well… I'm sorry. I do not mean this post in a harmful way. I didn't intend to disrespect opinions and if it looked I did at any point then I apologize. (That does not apply to the troll I had mentioned earlier. Assuming something is unusable just because you don't like the genre of mons… no.)
The main point of this is that, there are many cool ideas for inanimate object mons that could be done, so it'll be inevitable that there would be a lot of them in the future, so I just want to point it out.