If we find an alien bug via a meteorite, then that bug was one of the reigning champions in terms of ability to survive and subdue its opponents. It would have the capability of subduing our entire ecosystem leaving humans scrambling to try to remove it, contain it, then adapt to protect against it.
Look at Earth's extremophiles, the bacteria that can survive near thermal vents on the ocean floor, extended trips in space/vacuum, intense radiation, sub-zero temps, etc, there's no evidence that they're taking over the non-extreme locations. Even if they do survive outside their "home" environment, that doesn't automatically make them the top dog that will edge out everything else.
I'm not worried about fossilized bacteria taking over Earth's ecosystem.
That doesn't mean it's some kind of super-bug. The dodo survived long enough and I'd hardly call it a reigning champion. In the bacterial sense, there are plenty of harmless bacteria as well.