Earth to be destroyed

I think it is unlikely there were too many seeded planets. The number of potential habitable planets in the galaxy is only a tiny fraction of all planets and the likelihood of other planets with similar conditions to earth is infinitesimally small. Therefore earth would have been of high value to the engineers. I think they were planning to destroy us as they didn't like how we had evolved and perhaps felt that we were destroying the valuable ecosystem of earth and wanted to replace us with other more benign life forms.

The Engineers created humanity only to be host of the black goo. After thousands of years of worship they decided its time humanity would end by replacing them with more highly advanced beings - Xenos. That's why they were carrying the urns with the black goo. In my opinion both engineers and xenos want us dead. Predators are the ones on our side.

The engineers did not create humanity. They were bred from humans to be servants of a greater being. The xeno's are formiddable creatures, which like to live underground or move about mostly at night if ever above ground. They are "cave monsters", from some hellish world. The likelihood of there being a great many Earth-like planets just in our own Milky Way is not actually so small. Even if there is only one in a billion, there would be more than 100 at least.