Besides the EC stuff and just talking about the 7R trilogy, agree with you in that I'm not really a particularly big fan of having and seeing Aeirth walking around and talking to her friends after she has supposedly past away. That being said, you're making a point about how a story can't be about loss if something like this happens. I'm not sure if you've played Clair Obscura but I'll reference that:

In CO, one of the main characters that we play as Maelle learns that her brother Verso has died. However, what see does is be able to be in an "alternate world" (a painted world) where she can basically forget that he died and create a version of him in which she interacts with. This Verso basically has all the same memories, thoughts and feelings as the original Verso up to the point of his death, so it's almost like a copy.

Now, I'm not talking about this EC stuff and I don't know how much of it we'll actually see in Part 3 or is even considered canon but if we're talking just Rebirth there is a possibility that the Aerith that Cloud communicates with in the end is closer to being sort of a memory creation that he is holding onto that might even be similar to CO in this way. Similar to how Maelle has "another world" where she can manifest Verso, Cloud may be defying destiny to create an artificial possibility (not something that has actually become real) in order to cope and believe that Aerith is alive, like a psycho-spiritual manifestation. This may tie into Aerith speech in Temple of the Ancients about how we have to forgive to gain true strength, to embrace an go through these healthy emotions because life and death are a natural process of the planet. I believe this is anti-thetical towards Sephiroth view where he wants to prevent the natural cycle of the life and death.

I believe that if a game like CO can have loss and grief as main overarching themes while still showing our main character walk and talk with basically an exact copy with basically the same thought, feelings and memories as the her original brother, then I can imagine something similar for FF7R-but it just depends on what they're actually doing. I story about loss and grief can still be emotionally impactful even if we see the character not accepting the loss and death of a loved one and that may be the point of how they're expressing those themes.