The question "do we want to maintain a hierarchy of grief" is absurd, there is not one human alive who doesn't already do that on some level.
Grief is grief. What's at the top of your list might not be the top of someone elses. You don't invalidate someone else's grief because yours ranks higher on your own list.
It's just a silly attempt to sidestep the argument.
Our cat died two years ago and although we've had a few cats over the years, this cat was special. I was totally unprepared for the grief and pain that I, and the rest of my family members, experienced. I didn't feel like I could really even talk about it with people outside the family because it sounded ridiculous to so many people with the view of "it's just a cat!" But not this cat, and not to us.
My aunt died the same year. She was my mom's best friend and growing up I was at her house, hanging with my cousins, all the time. She was a total sweetheart: a kind, goodhearted, sweet woman with a heart of gold. When she passed I grieved, but although I feel genuinely ashamed to write this, the level of emotional intensity - the raw pain - did not reach the same level as when our cat died.
Grief over the loss of a beloved being does not come from a rational calculus of that being's worth.
I lost my dad at age 20 and in the months after was mostly surrounded by friends that had no idea what losing a parent felt like. In their attempts, I never felt I had the right to discredit their grief as being lesser, be it for an ex-girlfriend or a dog they grew up with.
I know that losing a pet hurts a lot more than losing a grandparent or aunt or uncle when they're not ever-present in your life. I have not lost parents yet, but at this remove in my life they are in many ways just people I used to know - my personality and values are quite different to them and I find it hard to empathize with their life choices. I think losing them would be similar to losing the little kitten I reared from a few weeks old. The actual relationship with my parents is mostly dead already, because it's been decades since we were close.
It's not about losing a human Vs losing an animal. It's about losing a relationship that is part of your daily life.