Mars is getting contaminated in the near future, whether we find life on it or not. Whether we collectively want it to be or not. Given the ever-decreasing cost of launches into space, the increasing access to small satellites, I think suspect it's inevitable that some collection of Earth microbes will get a ride to Mars within the next century. Whether it's a motivated team of grad students, an obsessed billionaire, or a small nation that doesn't decontaminate its lander before launch, it's going to happen.