Sorry, but somehow i 'don't got your point. Did you really mean, if in a TV Series there is a "Guest Star", with a limited amount of episodes in a Season, that his Story is not as interesting for you, because you did know, he is not there for ever?
That is really not my opinion. In the theory, a guest character could be exactly as interesting, as a permanent character. Sometimes he could much more interesting.
The problem is: It is a good story element. But because of the loss of a main-Charakter it is a bad gameplay element. And if i had to decide, i think Gameplay > Story.
It is true, if you say, the story was great. But FF7 is FF7, and that mean's it is not the only game, in which a main-Charakter dies. This is at first, a forum for selfmade-RPG's. And if i don't want to play ever again an RPG in which a Main-Charakter dies, i think, it is ok, to say that. The Final-Fantasy7 story may be really strong. But it is not a "law", that dying main-Charakters every time leads to a strong story.