Seems unlikely that a microbe that is adapted to the Venusian environment would outcompete microbes that are adapted to the Terran environment
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.]
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).