Seeing the ignorance and the political nature of most of the reviews on this episode, one can only come to the conclusion that humanity would deserve its fate when the environmental damage we all do on Earth will become critical. Too bad those responsible for the mess created won't be alive then to suffer the consequences, but their descendants will.
I didn't expect this kind of opinions on this episode, especially since TNG primarily speaks to intelligent and open-minded viewers, who would supposedly understand that if you leave your trash in front of your home, it will eventully stink and produce all kinds of bugs and other... "inconveniences". It's the same with pollution on Earth and the same with the effect of the warp drive on normal space in the fictional TNG universe. By definition, a warp drive is warping space to move an object instead of increasing the kinetic energy of the object, by generating warp fields to form a subspace bubble that envelops the starship, distorting the local spacetime continuum and moving the starship at velocities that could greatly exceed the speed of light. It's similar to folding a thin piece of metal, to get one side next to the other. Now, even children know what happens when you quickly and repeatedly do this to that thin piece of metal: it eventually breaks - just like paper is easier to tear in a straight line after folding it and the same as space does after being repeatedly "warped" (i.e. folded) in the same location(s) by starships. Bottom line, there's nothing "preachy" about the scenario in this episode, just like it's nothing "preachy" about the fact that pollution or deforestation are endangering life on Earth - it's simply the truth.
So, what can you do faced with these close-minded, bureaucracy oriented and irresponsible people, who would either not believe or not take or delay any measure to alleviate or correct the damage? You'd do what the alien's sister did in this episode, sacrificing safety for understanding and appropriate reaction. Maybe the reviewers who considered this fine episode "preachy" need to experience the same - there's nothing more convincing than having to fight for survival. It's up to the viewers to connect the dots and realize the similarities between what happens in this episode in a fictional world and what already happens or could happen in our very real one. Unless they're blinded by petty concerns like profit or politics, that is...