But what "proof" is acceptable?

Nothing so far adduced has been able to meet the ever-shifting requirements of "proof." Therefore the Sterile Mars proposition still holds sway.

My point is that Sterile Mars shouldn't be accepted at all without affirmative evidence to preclude biology. There is no such evidence. There is evidence to limit and restrict terrestrial biology under Mars conditions, but that says nothing whatsoever about native Mars biology (if there is any). There is evidence that may restrict or limit the development of native Mars biology under current or hypothesized past condtions on Mars.

And there is an abundance of evidence that the surface of Mars provides many opportunities for native biology to survive (or even thrive according to Mars-specific standards.)

I am calling for a leap of logic to get past the now decades-long impasse over how to approach the question of life on Mars.