“If they can live in such harsh environments the surface world could be a bonanza for them and a disaster for us. “
Actually no. They have adapted to this extreme harsh environment and that is where they thrive. Take them out of that environment and they die as the new environment is harsh to them. A perfect example is the Thermopylae bacteria that live in the boiling springs in Yellowstone park. Take that bacteria out of the springs and it dies.
Actually we have been exposed to these bacteria over the eons. When great meteors hit the earth ejecta from the impact scattered material from deep within the earth over the surface. Do not worry about the bugs deep in the earth.
Click the below link to see the effects of a large meteor impacting the earth.
https://www.livescience.com/56914-dino-killing-asteroid-punched-through-earths-crust.html
These changes may have proven critical for the evolution of life on Earth, and perhaps on other planets, Gulick said. "When you get rocks with 10 percent more pore space, microbial life living below the surface may find new habitats on the surface," he said. "Our next area of research involves looking at whether ecosystems can get started by craters."