I'll preface my answer by pointing out that you concede it boils down to a matter of convenience. It's inconvenient to "celebrate Hollywood", as you call Once Upon, just as it's convenient to be politically demagogue, as I think is the case with Parasite.
I don't care about the implications of either movie getting picked. I don't care how it would "look" to reward Tarantino's movie. The award is Best Picture, one of them was better than the other, and Parasite (which was fantastic by the way, bar muddled ending) wasn't it. I would've given it Best Direction and Foreign Movie alone. Maybe even Screenplay, since I think Once Upon a Time's isn't Tarantino's strongest. Best Film though? Not for a second.