> contaminant microbe would have to truly thrive in an extremely inhospitable Martian environment, survive spaceflight and decontamination, yet also be undetectable in the places on Earth where the spacecraft was constructed

To be fair, if those possibilities are true, they would also be revolutionary and should be considered (e.g., do they imply a mechanism that makes panspemria feasible?).

Likewise for an previously unknown mechanism of methane production - this itself would help us understand the origin of life. (This is one explanation, "geological" one, mentioned in the article.)

"Single celled microorganisms found to exist in conditions on Mars that mirror some of the conditions in which single celled microogranisms exist on Earth", however, seems to be a more boring one - if turns out not to be true, understanding why that happens to be the case, is also valuable.