As a collector of anything one would consider rare, I wouldn't really like the idea too much.
It may sound selfish but yea, collecting is usually a selfish thing, and I like when some things keep their rarity value. It just feels better when you have something not many others do (at least to me), and it feels more of an achievement when you get that rare something you wanted through patience and perseverance. It doesn't really feel as great if it's something everybody else could easily get, and it would no longer be considered rare.
Well if something like this does happen then that's fine with me. Pets aren't game-breaking like you say anyway so ultimately there's no harm into doing so. Just gonna miss not seeing plenty of people have my pets