Far from the most parsimonious view. It seems absurd to invoke an alien origin for life on a world as conducive to it as ours.
Google "panspermia". It's hardly some new, radical idea (having been referenced since at least the 5th Century BC). At this time, it's just a theory which may be expanded upon or dismissed altogether as new evidence and new theories are born. Therefore, panspermia is still under investigation.
If you mean a different, indigenous intelligence, barely plausible.
Based on what? People have been reporting the same things for as long as there's been people to report them. This doesn't necessarily mean an "alien" life is/was here but there may be some grain of truth to all of the sightings that have always and keep being reported, even if we don't have the ability to understand what that
commonality might be. I see the same grain of truth in many of the creation myths told the world over. For example: I don't believe there was ever a "great flood" but it's generally accepted that the world was once entirely covered in water. Early civilizations which saw sea shells in high and dry areas (such as mountain tops) might develop a philosophy or religion based on a "great flood" because, for them, that was the best answer they could find. Apply the same logic to all the sightings and it could mean anything from extraterrestrial/inter-dimensional life to some other unrecognized life or simply the affects of hallucinogenic plants on certain brain chemistries.
Of course we weren't always here; our species is about 200,000 years old.
Yes, dear. That's part of my point. With the earth being about 4.5 billion years old and mankind only being here for the briefest part of that time, could something else have been here first?
Sure but unfortunately, resources can be much harder to exploit on uninhabitable worlds.
How are you reaching this conclusion? Arecibo's Planetary Habitability Laboratory (PHL) just released a
list of five worlds which could potentially be inhabitable but as far as we know, does not (yet) have life. For that matter, some planets which could not currently sustain life on its own could easily be mined for natural resources. Mars is red due to the iron oxide on its surface. Mars also contains silicon and other metals. I therefore don't understand why taking resources from these planets would be more difficult than taking resources from earth. You're also making the assumption that any alien life would never have come upon any other planets, probably much further away from earth, which could have whatever resources they need.
Mercurian, lunar, Martian and jovian water, for example, is almost entirely frozen. Btw there are many reports of aliens taking water and various other things.
And that means what to an alien species with alien technology that may be thousands of years or more advanced than our own? You're also ignoring the planets we've already found which may possibly be inhabitable and may have at least some water. You're placing human constrains on something which may not be limited by human technology or understanding. It's possible if something was advanced enough to travel the galaxy or further, it may well have found plenty of other resources to exploit.
I think their reasoning would be comprehensible, since rationality is the key to progress. Despite the vast technical gap between our world and that of fifth century BCE Greece, there was much in the latter we'd recognize--propaganda, alliances etc.
You're still be limited by human constraints. What is "rational" to us may be irrational by a more advanced (or less advanced) species. We don't even have to travel of off earth to see this. What is "rational" by one culture may seem entirely irrational to another. This is often determined by a culture's needs, experiences and technology. What was "rational" to the Conquistadors was irrational to the Mesoamericans. What is "rational" to the Military Industrial Complex may seem entirely irrational to those who think cooperation and the right to self-determination is the key to human survival. What is "rational" to those who exploit the earth without regard to sustainability is irrational to those who see sustainability as necessary for earth's survival and the survival for every species on earth. Some people see religion as the only "rational" explanation to life's questions. Others treat religion as a philosophy while still others think anything hinting of religion must be, by definition, entirely irrational.