"You can be paired up with the sweatiest player possible"
If they're the sweatiest player possible, they'll be moving up and playing up in GP, and not your scenario.
"That's not fair in the slightest"
Again, if they're where they should be, it is fair. If they've fallen due to inactivity, you're right it's not fair. If you play all the time you're where you should be and playing who you should (bar those that aren't playing as mentioned). Just having a GP metric isn't the answer.
I saw an example of a 9 million GP account making Kyber 1. What happens to this player if there's a GP match making metric that doesn't allow them to play vs 13/14 million accounts?
There's zero issue with players who have moved up playing larger accounts or playing accounts that have fallen due to not being very good. It's a case of wanting to win more, it's not set up like that. You'll win 40-60% of all your games, apart from when you make a meaningful change to your roster. If you can't accept winning 50ish%, then you're in for a lot of pain.