I see the plot of a sci-fi story where scientists bring home some of the Venusian microbes for study and accidentally wipe out all life on Earth after it escapes the lab, multiplies rapidly, and Earth has no defense. Or the microbes could be all that is left of a doomed Venus society that genetically
engineered these microbes to lay in wait in their atmosphere for the day that a curious space traveling society comes to investigate only to be parasitically taken over as a host.
Come to think of it, I think Asimov already wrote that same story in "Green Patches".
Everyone uses "Jovian" for Jupiter, and yet nobody uses "Venerean" for Venus...
[The planets take adjectives in -an because, I assume, people didn't want to reuse "Martial", "Jovial", and "Venereal". In science fiction you also see "Terran" replacing "Terrestrial".
Interestingly, while martial and venereal are named after the gods, jovial and mercurial are named after the planets.]
When wood-like structures first evolved there were no bacteria that could penetrate the tough cellulose. The trees would just fall over and ... stay there, as rotting wood is just wood being eaten by bacteria. This is where coal comes from.
I suppose something along those lines could happen: Venusian microbes are sufficiently alien that Earth-based life can't really interact with them, while the Venusian can interact with Earth-based life, or something like that. They can eat you but you can't eat them.
In lots of sci-fi like Star Trek all aliens have DNA and you can create Vulcan-Human hybrids and whatnot, and while e.g. Star Trek does offer an explanation for this[1] in reality alien life would be foundationally different. It will have some sort of DNA-like encoding structure, which isn't DNA.
I agree it's somewhat unlikely, but it's also completely uncharted all-bets-are-off territory. It's not so much about competing, but more about how it will interact (or not).
Come to think of it, I think Asimov already wrote that same story in "Green Patches".