We don’t know that.

We do, because the opposite effect took that long. It’s likely even worse for the reasons mentioned.

we’re already fucked because whatever faction we might take today should’ve already been taken decades ago.

That’s true either way with where we’re at. That’s why we call for drastic actions to be taken, especially since governments can’t even agree to implement what’s asked for by scientific advisors, who are already very conservative in their predictions in order to not push those politicians too extremely.

If a global humanitarian crisis is mere decades away, no changes we’ll feasibly make today or in the near future will stave it off.

That’s not correct, because it can always get even worse. The more and sooner we get rid of our emissions, the better are our chances. That’s also why, on a fixed time scale, it is important to do the bulk of the work as early as possible, instead of doing it towards the end. The longer those greenhouse gasses are in the air, the more damage it will cause for us in the long run. But right now literally all of our measurements taken are still causing us to shoot far beyond our set targets (which turns out, were already too conservatively set too).