Mirror proteins behave much like their natural counterparts, with one important difference: They take much longer to break down. That’s because the natural enzymes that normally degrade proteins have shapes that are adapted for attacking left-handed proteins. They cannot grip mirror proteins and cut them into fragments.

The virtual non-existence of enzymes that can break down right-handed proteins is almost assuredly because their use is vanishingly rare in life on Earth right now. If mirror life did escape the lab and find some way to thrive, normal life would suffer until some normal bacteria happened to mutate and create enzymes that could break it down.

I expect it to be like the Carboniferous period. Trees evolved, and nothing was around that could break down lignin, so they thrived for millions of years and caused devastation to ecosystems of the time. But dead trees represented a lot of untapped raw materials, so eventually other life evolved to break them down.

I would expect the same with mirror life. All else being equal, a few million years of devastation until life evolved ways to fight back. Or humans could dramatically speed that up by genetically engineering normal life (bacteria) with the tools to break down mirror proteins and thus attack mirror cells. It would still be devastating and would completely reshape life on the planet, but it may let humanity squeak through and continue existing.