In reality, there would never be an economic reason to travel to another star.

Travelling to Alpha Centauri would entail enormous technical and engineering challenges. The level of science needed for interstellar travel far surpasses the relatively simple task of synthesising a mineral. Whatever properties unobtanium has, it's simply protons and neutrons and electrons, which can be manipulated in a laboratory for 0.00000001% of the expense of going to another star.

The analogy would be something like Europeans travelling across the Atlantic Ocean for something trivial like clean water, when it's much easier just to filter the dirty water at home.

Therefore all the alien invasion scenarios are false e.g. they want to eat us or want our land. Because if you can travel to another star, you've ALREADY SOLVED all other engineering challenges.