My understanding about planetary protection is that you don't want to contaminate the new environment, because then you can't say for certain (that easily) if the life you are detecting is native to the environment or the contamination.
I also understand that the native population of the Americas suffered badly from the diseases carried in by the europeans. But those diseases landed in an environment much like the one they evolved in. The humans were humans, the atmosphere were the same, the temperature were the same, etc.
If the space-probe gets contaminated with some earthly bacteria, virus or prion they not only have to survive the travel there, but then they have to quickly adapt to the new environment. Different temperature, different chemical composition, different life forms. How often does it happen that you sneeze at a fungi and the fungi gets infected with your cold? Doing one of these adaptation alone is a big ask, doing all of them at once would be a huge leap.
Imagine that you move to a new place. You move there in a salted barrel, not designed for human occupancy. The locals speak a different language, have different customs, you can't get food you are used to and the climate is way out of your comfort zone. Would you be outcompeting the locals quick? I don't think so.
I wouldn't worry about wiping out a whole ecosystem unless the ecosystem is much much more similar to our own's.
But of course being cautious is always a good idea. Furthermore I already think we should avoid contamination. If for no other reasons than to avoid arguments about what the detected life really means.
