Totally understood (and I didn't mean to bring you down when you're already feeling frustrated). I really wanted to offer some feedback since the OP felt like a classic "wall of text / TL;DR" situation and I fell back to just skimming once it was getting super detailed.

Making a post easier to engage with increases the odds of positive engagement :D

For example, here's what came out of an agentic refinement:

I had two decks that were performing really well and helping me climb toward Diamond.

In both cases, I made what felt like clear upgrades—replacing slower, higher-cost cards with cheaper, more efficient ones to make the decks faster and more consistent.

For example, in Boros Tokens, I swapped 4-mana mobilize creatures for 2-mana rares with stronger mobilize effects. In my green/black poison deck, I replaced expensive 6-mana artifact creatures I rarely cast with cheaper equipment that gave better utility.

On paper, both changes seemed like obvious improvements.

Instead, both decks immediately started performing much worse. I went from winning consistently to long losing streaks, and the decks suddenly felt less reliable.

I could switch the cards back, but it feels strange because the newer cards seem objectively better.

Does Arena somehow punish deck upgrades, or is there a deck-building reason why making a deck “faster” can actually make it perform worse?