My disbelief in UFO's does not preclude me from believing there is life somewhere else in the universe, which I do.
My problem with UFO's is simply a matter of physics. The distances to traverse the span of empty space, even from the most nearby sun systems, is just to great to overcome by any means we're aware of.... in fact the distances are so great that it's next to impossible to envision even with a vivid imagination.
Some speculate that travel times of millions or even hundreds of million years is possible... but as we know things that's not likely. We know there is life on earth and we also know the universe isn't old enough to have allowed the time needed for life to evolve to where we are technologically, must less very advanced of where we are.
Also... why? By far the most reasonable alien contact would be some type of 'signal', not actual transportation of the life form itself.
And yet, none of that stops me from 'wanting' to believe it, or hoping it's somehow true.
Bigfoot, Nessie, Yeti... and the rest, I don't think exist today.... but we know man existed with Mammoths who no longer exist. So some type of bigfoot creature in the not so distant past I think is reasonable to believe in. Stories could have been handed down... based in fact.
So although I think a current Bigfoot extremely unlikely I do think it far more likely than Alien UFO's visiting earth.
That's all just me, obviously... like you the topics fascinate me and I'm always eager to listen to the next theory that comes along. - JamesJM