are we really arguing over "how high up on the list of cloud's reasons to kill sephiroth is aerith's death"? because this is honestly really pointless. how the fuck are we going to determine whether the loss of aerith is more or less significant than the loss of his mother or a bunch of NPCs? we will never truly know cloud's motivations or priorities, we can only go off of what the game tells us.
i think the important thing here isn't exactly how important the loss of aerith was vs. the massive amount of loss that cloud has suffered in his life, but how her death affected his course of action.
whether they were porking or not, her loss was what finally, truly motivated him to bring down sephiroth once and for all. sure, maybe it was just a straw that broke the camel's back. or maybe, as AC seemed to hint at, it was a huge blow to cloud and it was what gave him the strength to fight back. does it really matter if it made him more or less sad than losing zack, or his mom, or the assholes that lived in his hometown? who even cares, it's not fucking relevant to the story in any way shape or form. the only reason we're arguing this is because it gives you a reason to downplay aerith's role in shaping cloud as a person, which we have all established as INCREDIBLY SILLY.
so yeah, maybe it's not "the reason why he killed sephiroth" or "the definite most saddest thing that ever happened to him", but it was what caused a turn in cloud's course of action, and that's what matters. also, i'm pretty sure this is what zee meant all along, but i guess reasonable deduction just isn't anyone's strong suit.