I think that's pretty reasonable, but if martian life has a completely different biochemical basis, it might be much less able to adapt. So, maybe, it has managed to hold on for a billion years, but terrestrial microbes may come equipped with a much richer genetic library, allowing them to adapt exponentially more quickly and overrun martian life.
Another way to look at it is that Mars seems to have almost no biodiversity, so it's possible that martian life forms may be well adapted to their environment, but not for competition. Terrestrial microbes have evolved in an utterly brutal competitive environment and they've been shown to adapt to extreme conditions, so it's not unthinkable that they may have a substantial edge over martian life.
Maybe. Just a laymen, so I could be totally wrong.
Another way to look at it is that Mars seems to have almost no biodiversity, so it's possible that martian life forms may be well adapted to their environment, but not for competition. Terrestrial microbes have evolved in an utterly brutal competitive environment and they've been shown to adapt to extreme conditions, so it's not unthinkable that they may have a substantial edge over martian life.
Maybe. Just a laymen, so I could be totally wrong.