The caves and small places of Earth say otherwise. So do the intense environments on Earth that we've found life in. It's at least as plausible that life can hang on, sheltered in the smallest of places, as it can anywhere else. Micro organics often don't mind if you put them in a small place, with little sunlight, high or low heat, high or low oxygen, etc.

The biggest problem would be if the changes Mars underwent were too fundamentally anti-life for anything to survive whether it was in a cave or anywhere else.

In my opinion, we're going to find out on Mars that there are forms of microbes that can hang on in sedentary 'ready' state for a billion years, in extreme environments, just waiting to jump back to activity. Most of Mars will be barren, but there will be small pockets of microbes (most likely in a sheltered cave, under ice, or similar) in a form of extreme suspended animation.