I was a 5k hp Sion

I thought you were gonna say "And then the other team built LW and 2+ BotRKs"

You forgot, "Sincerely: every last player who doesn't understand the balance issues of Atma's Impaler."

First problem; it's offense-defense. This makes it more difficult to balance by design, because it can't be stronger than a straight offense or straight defense item in that regard, the combination needs to be only situationally a better option than straight offense or defense, and, at the same time, it can't be significantly weaker than the other options. It's being compared to a much larger item pool than other options. That's difficult enough to balance; then, there's the fact that it's AD and Armor. When Atma's was viable, it was just a race to see who builds it first, because there were few things you could buy early on for that much gold with as much efficiency. "But there's AP/MR items, and those are fine." Different stats, different champions, different playstyles. AP/MR is balanced because they tend to be poke or burst, rather than consistent DPS. AD/Armor is harder to balance, because you are both increasing your DPS (decreasing your opponent's lifespan) and your defense (increasing your lifespan) by stats that are only going to be more valuable late-game because of multipliers (AS and HP, mainly).

Second problem; it's offense scaled on defense. In general, permanent stat conversions has been messy for balance. The worst have been offense to defense or defense to offense, such as Jax's old passive and Vladimir's passive, because Fighters are balanced around the concept that their overall effectiveness increases whether you build offense (deal damage faster) or defense (live longer to keep dealing damage). So, when you're able to scale both from buying one or the other, you can end up dealing damage almost as well as the person who built only offense, and surviving almost as long as the person who built only defense. Why risk being a glass cannon who needs defenders? Why risk playing a low-damage tank? You can be a high-damage tank with Offense-Defense and Defense-Offense conversions.

Go play 3s... it's still there XD