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.