We are not alone imo. But what are the slimist chance we were visited by something smart enough to fly here and, hell, wreck and be by themselves...
The world were smart enough to makes planes that could fly over remote islands(where people never saw white people before). Those planes were amazing techs, but planes still crash.
Ufos most likely crash, as when existing in our reality, they have to conform to rules in that reality. That means some energy has an effect on them, there has to be some freq that will effect them, and thats why radar probably was responsible for bringing them down. It was a fluke by humans to find the right freq, but i assume they did.