Saying that the Lifestream is made of the energy of memories isn't the same thing as saying it's a collection of distinct, discreet memories. Memories are not 'the building blocks of the spirit'. Memories are the building blocks of
identity, as this game clearly shows, or at least it used to. The Lifestream isn't just a physical fifth element that flows perpetually around the planet. It's also inside every living being. It flows into them at conception and out of them when they die. It's "命", inochi, which (according to Denshi Jisho) translates as both 'life force' and 'destiny'. Identities and their unique memories dissolve in the Lifestream after the death of the physical body. If they didn't, every living entity would be able to remember all of its spirit energy's past lives as a tree, a worm, fish etc... (Unless we imagine some convenient apparatus exists to circumvent this logical conclusion.) The Lifestream is the will of the Planet, and it is the will of the Planet to create life. Individual lives come and go; life is eternal.
If people like the kind of thing FFVII is turning into, that's great. I am, however, allowed to dislike the focus on timey-wimey stuff without being told that I'm somehow insufficiently appreciative of the elements of fantasy.
The fact that other FF have well built worlds doesn't entail the conclusion that the FFVII Remake does also. It may eventually turn out to have one, but the jury should still be out right now.
If the Lifestream flows between life and death for every creature that's existed on the planet, and that flow constantly accumulates memories from beginning to end, meaning then those memories are linked together. The Sephiroth is able to reach back from the Edge of Creation back to the present and interact with Cloud and the others isn't a lie. That's real. Sephiroth is there at the point the planet dies, and that moment in time is able to be reached thanks to the Whispers.
I can appreciate that you and many other fans are very excited by this concept, but to me it's completely underwhelming.