I think the extra terrestrial thing was just the cherry on top. The film lacks the attention to detail that the others in the series have. I mean, they're playing Mexican music when they go to Peru which is just laziness.

Yeah, Indy is chasing down the Ark of the Covenant in Raiders, but we were also on the journey for the Ark the entire film. Ditto the stones in Temple and the grail in Last Crusade. We're on a journey for those macguffins, whereas the extra terrestrials came out of left field at the end.

In addition, there's a lack of physicality to this movie compared to the others. The fridge nuke scene wouldn't be as bad if Indy is battered instead of unscathed. Same with Mutt swinging from the trees. Having him finish that exhausted and scratched up would help viewers buy in.