A huge amount of carbon is emitted by sources that can't be profitably replaced in the near future: coal (especially in China), concrete, and cars come to mind.

The only entities that can accelerate the switch is the government. Those sources of carbon are still too cheap in most places for consumers or businesses to want to stop using them entirely.

I'll also point out that OP asked what a software dev can do, not what a physicist or engineer can do.