Most of these contamination concerns are only relevant while we're still at the robotic exploration stage. When humans inevitably set foot on the planet in a co

Most of these contamination concerns are only relevant while we're still at the robotic exploration stage. When humans inevitably set foot on the planet in a couple of decades most contamination concerns become moot since it's basically impossible to contain the amount of life that comes along with any animal, especially when you have the systems to sustain those animals for years. It's a somewhat ironic situation I think, since with how limited our ability to explore is with robotic probes and how limited even the sample return's sampling area would be, the only serious shot at actually ruling out the potential for native Martian life would come with a years long human presence (as would be needed due to the 2 years between arrival and return windows) and the associated ability to more thoroughly explore - either in terms of actually having people going around and exploring or in terms of just being able to be more flexible in performing experiments on samples brought over by robots. In a way the sample gathering (and potential return) is also in preparation for this inevitability. NASA is well aware that if the plans working up to boots on Mars materialize, having 'pristine' reference samples will be pretty important, so even if they can't bring them over to Earth yet, they can at least preserve them for later reference.