if it did I would have thought it would have appeared in the past 5 billion years or so
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.
I am not alarmed by the possibility of mirror life because I think it would be at a disadvantage to all other life on earth at present, so it probably wouldn't get very far. (famous last words!?)
It may have appeared and been outcompeted. That doesn’t suggest its (artificial) reintroduction would again be outcompeted
Now, with L-amino life everywhere it would have an even bigger starting problem.
We see this with extremophiles: organisms which can grow in nuclear cooling ponds full extremely badly when transplanted to nutrient rich environments with other, less capable organisms.
