I switched to Artifacts at the start of the set, got dumpstered, went back to my Storm Haven without changing a single card from the first set since I didn't pull anything and started winning again.

People seriously underestimate how much skill expression is involved in this game, how hard it is to pick up new decks if you're starting out at a higher rating, and how much comfort matters in your deck selection. I think a lot of the cases of people losing this set after being fine last set is the same situation as me where they changed decks and completely underestimated how much learning there was to do. Tiers are real and your deck matters, but it really doesn't matter nearly as much as your ability to pilot it right now. Tiers simply aren't very far apart right now.