It is ironic that the "father" of the Ancient Aliens theory, Mr. Erich Von Däniken, is the last person on earth who would sell his theories as "hard science".
In fact, having seen him in person in my youth, as well as in several excellent TV interviews, Erich will not get tired of pointing out that it's on the watchers and readers to come to conclusions on their own.
He keeps asking questions, instead of saying "it is so" as we see in the so called "established" sciences. He will ask questions like "could it be that..." instead of claiming that his theses are the last word. Heck, he even admits that he made errors in the past.
Erich is right too in pointing out that it is very often the established sciences (for example Egyptology) which is stuck in and bound by dogmas - in essence meaning that science is also more often than not a belief system, with scientists being the "believers".
The Ancient Alien theorists and "believers" make it clear and never denied that their theories are, well, speculation. Speculations which are indeed healthy because they encourage to question scientific theories and dogmas which are written "in stone" for centuries already.
There is nothing wrong with that.
Only if you ask such questions and accept alternative views you are indeed conducting real and true science.