However, if the model is accurate, then warm and wet conditions on Mars were never stable enough to provide for the proliferation of microbial life at the surface even if it emerged below the surface. This would make extant or extinct life all the harder to find there.

As for life being found in extreme terrestrial environments, keep in mind that these extreme places are still inextricably, if remotely, linked to a large and active global biosphere. This is a condition that Mars has apparently, never possessed. So while its possible to find comparable extreme environments on Mars, they do not benefit from even a remote association with a more life friendly environment.

I would love for there to be evidence of either extant or extinct microbial life on Mars. But I am deeply pessimistic that we will find any there.