Haven't read the whole thread so apologies if I'm repeating this but :
Allowing even low estimates for the number of galaxies / suns / planets in the 'Goldilocks Zone' / life starting / intelligence makes it probable that intelligent life evolved elsewhere. Factor in FTL travel (which is a stretch) and you could still get theoretical visitors.
None of this, however, takes into account the staggering timescale or 'deep time' since the Big Bang. The chances of one of the aforementioned 'visitors' civilisations existing at the same time as ours is very very small indeed.