HG Wells is right, we best beware of plot twists
The idea that earth life forms need not worry about off earth life forms because life here has co-developed is flawed, so much so that I think most can see why if they thought about it.
We have only one example of life and can only take educated guesses at what other life forms there could be. Having only one example when there may be many other examples should not inspire confidence that all are similar.
Even if one considers that life forms of similar kind would be contained by distances between systems we should be cautious based on the characteristics of life here on earth, in our own system.
When life here experiences a major disruption it tends to oscillate between extremes, with one life form dominating, then collapsing as another grows to dominate and so on. It takes some time, some long time, before a homeostasis, or equilibrium develops making the system more stable for all remaining life.
Any new life form could cause a major disruption. Our current success as a life form is dependent on earths ecosystem. That is changing and hopefully it will not be too long before we are not so dependent but until we invent free energy of energy "too cheep to meter" we are very dependent on earths ecosystem.
A quick thought of the risk analysis shows that the negative outcome of not controlling the introduction of foreign life forms, at least when we can, is so great that it must be addressed.
At least more so than a confident "Sorry it wouldn't happen".
Although in my experience most people prefer such false confidence and will promote the confident fool offering empty assurances (not that jmch is a fool I thinking of you Bob). Further promotions follow when catastrophe strikes and the same fool (you again Bob) jumps up to claim no one saw it coming and then uses the procedure developed by the person who not only saw it coming, spoke up at cost, and then developed a response just in case. But I digress.
We can't know, the consequences are high, caution is warranted.