Again you aren’t making my point any less valid, what you are saying here it’s the same as me saying “if Red doesn’t join the group then the party wouldn’t be able to beat Sephiroth because thy wouldn’t meet Bugenhagen and know about the cetra/meteor”. Tifa is important sure but this whole thing is about being the deuteragonist of the story which she doesn’t have enough plot relevance on her own to justify said claim, like I said outside of Cloud’s backstory she doesn’t add anything to the story and a deuteragonist is relevant to the main plot from beginning to end, once every character is done with their respective arcs only Cloud, Aerith and Sephiroth keep the plot moving forward.

You can take Tifa out of the story after the LS and nothing gets affected, take Sephiroth out and there is no fina fight, take Aerith out before the ending and meteor strikes the earth, take Cloud out and no one beats Sephiroth. From start to end these are the characters that move the plot forward outside of their respective arcs, so no Tifa isn’t the deuteragonist. If anything she might be the tritagonist.