On the other hand, for all I know, anaerobic life originally evolved on Earth at the undersea rifts.

I see either result (life, no life) on Mars as potentially good news.

If there is life, we will have learned something very important about our solar system, and the universe in general. If the life has genetic tracks that are similar to Earth life, then either evolution runs a much tighter course than we think, or the panspermia folks will have something to brag about. Assuming it is established that Mars life does show evidence of not having evolved separately on Mars, and given enough analysis, it may even be possible to ascertain whether Mars life came from Earth (possible), vice versa (less likely), or even more interesting, both came from a common ancestor from deep space. ANY positive results will give us huge, paradigm-changing insignts.

If there is no life, and no sign that any was ever there (How would you prove this, short of sifting the entire planet? How do you prove there are no bacteria ANYWHERE on Mars?), biologists will be disappointed, but it means that the planet is ours for the taking. Line forms to the right.