After my experience I am very optimistic that there are greater things awaiting our discovery.
You do make a very valid point about how technology, that what may seem impossible today can be routine tomorrow. Sure aliens are possible. Believe me, as a child that voraciously read all the books on UFOs, Nessie etc., I would love to have confirmation of ET visitations.
But then, humans making remarkable advances and finding keen ways of using them is a repeated certainty. So I'd have to go with my head and say that experimental craft is the most likely explanation in many sightings. I want room temperature superconductors!