I would say 90% of my collection are pets I adopted c: and I am from 2009! Take a look:

Dog adopted on my join date!

A Halloween pet I token hunted for!

My first 12/18 pull!

My first pet that ended up a rarer outcome!

First summer event pet!
I would say there were a few factors that allowed me to keep so many adopted pets. 1. I was terrible at trading. I was a kid. I didn't know anything. The few trades I sent were canceled ;.; apparently I had "trash pets" (no 08s). 2. I had this rule, for myself, I would only trade a pet if I got a double of a double. Meaning. 3 copies of the same pet. 1 for my collection. 2 for for my trades group. If I only have 1 outcome extra in my trade group I would make an excuse to never trade it "well I only have one". I didn't get over that logic til 2019 probably.
I have since disbanded some pet groups. I gave away my tess cats. And my tess dragons. The rest of my pets I am happy to have them. They are living memories ^^ priceless! The doubles though, I actively try and trade those or give them away during holibombs or lights on Lights off.
I am always curious if players keep the majority of their adopted pets. I see a trend now of c$ shops, so it wouldn't surprise me it there are players with little to no originally adopted pets on their account! The opposite of my methods haha.