Cyanobacteria reproduce rapidly in sunlight and their numbers are mostly kept in check by viruses. If you bred a species that didn't have any existing viruses there would still be amoebas, flagellates, etc which would use the extra food to reproduced faster and keep the numbers in check, though it would be a big ecological problem. If these couldn't eat them because the new cyanobacteria was mirror life they they'd lock huge amount of carbon from the biosphere as indigestible sugars and starving all other plant life, over the course of maybe a year at the most.

It's possible that humanity could survive by exploiting things like fossil fuels but by default it would be as bad as the extinction that ended the Permian. And certainly most humans would die.

We're protected from this naturally because it takes over a billion years to evolve something as good at reproducing as cyanobacteria from scratch and any biogenisis that were to happen in the modern world would produce something so hapless it would be swiftly out competed for resources. You can't evolve from a regular bacteria to a mirror bacteria, evolution is really bad at making multiple changes at once and this would require changing literally every part of an organism at once.