In one degree, you are correct that if something is dangerous, we might find out too late. On the other hand, if results continually indicate minimal adverse effects, then, it gives reassurance.
The problem with labs and case studies is that they are short-term. The only option is to either (a) drastically overdose the subject, which means nothing (you can even die from drinking too much water in a short interval), or (b) report short-term effects.
There is a publication journal, "Inhalation Toxicology," which has done such studies, and they DO in fact report some degree of adverse effect in rats, with a lesser degree, almost negligible in Beagles.
This was not an ecigarette study. It was a PG vapor study. Probably not as prone to bias for that reason.
If I had to guess, based on the limited information available, we are doing some degree of harm. The pleasure derived from smoking and vaping both relates to the irritant. If there was no irritant in this product, it would be useless as a substitute.
IMO (not scientifically), any long-term, repeated exposure to an irritant causes damage. Hand soap is safe on the skin, provided you wash it off. Try leaving Ivory soap on overnight, and it will burn the ....e out of you.