After the whole debacle with recycling, I am more inclined to believe this is true.

I agree CO2 has become a red herring. But I believe a lot of intelligent people supported its use for a long time not out of malice, but because it was a single metric that, while not a very good proxy for the various pollutants that are emitted by large polluters, was probably the best single proxy available.

You are underestimating how much man made CO2 is in the atmosphere. Preindustrial CO2 levels were at around 280ppm. Nowadays we are at 415ppm and no sign of any slowdown. 30% of the CO2 in the atmosphere came from burning fossil fuels. We are just a few decades away from doubling CO2 levels compared to preindustrial times.

On the order of millions of years the earth is a self correcting system that always tries to release as much energy as it absorbs. If you disrupt it in mere centuries you're going to see mass extinctions. Humans can adapt but often that just means they migrate to more hospitable locations. It means more refugees than we already have. The negative effects on agriculture will make it harder to feed 8 billion people so eventually civil wars will break out.

We are already seeing mass extinctions though, and biodiversity is generally greater in warmer regions. Unless these extinctions are entirely due to the 0.9 degrees of warming we've seen, there are other factors at play, many of which might yield more bang for our buck if we focused on them instead of net CO2e.

I think of measuring environmental health via CO2 like measuring economic health via GDP - it's probably the best single indicator if we have to use a single indicator, but it ignores a lot of nuance without which a better system can't really be designed.