The idea that unintelligent life from another planet would be unstoppable on Earth, devouring everything in its path, seems ridiculous to me. All organisms are

The idea that unintelligent life from another planet would be unstoppable on Earth, devouring everything in its path, seems ridiculous to me. All organisms are really good at repelling foreign invasions, both inorganic and organic. The only pathogens or organisms that have any chance at all have co-evolved over billions of years, just to have a small chance at succeeding against a healthy target. Any alien microorganisms that find themselves released into the wild on Earth are going to be devoured. They won't stand a chance against the millions of existing species and their varied arsenal of weapons. They weren't here to evolve defenses. Some more sci-fi ideas like inorganic life or nanobots are a different story, but if we're talking about the equivalent of Martian fungi, I'm not worried.