Er...I think we have a fundamental difference in opinion about how video game storytelling works. It wasn't my story, it was Bioware's story. We're given a lot of agency within the series to make the choices we would like our Shepard to make, but we don't control the narrative structure of the game or, really, anything other than our Shepard. Shepard was set up as the central figure, and thus a lot of things hinged on what choice Shepard made. But there are some things in the story that have to happen no matter what the main character does or says. Either Ashley or Kaidan WILL die, no matter what you do. You can save one of them, but no matter what you do you can't save both. Shepard's death is exactly like that. You can have the known universe be as prepared as possible, but Shepard DOES NOT survive the final encounter. Shepard wins the final encounter, but does not survive it. Which isn't to say all the preparing you do is meaningless, because the better prepared you are the less damage the Reapers do to the galaxy at large. And I don't consider Shepard's death to be at all the same as DAO's Ultimate Sacrifice, because the Warden goes into the final battle knowing, without a doubt, that s/he is going to die if they don't do the Dark Ritual. Shepard is given no such assurance, or way out. Because one does not exist for Shepard.
Then we do have a difference in opinion. I do not consider the Mass Effect to be a novel, written by one person, but a game. Specifically, a role-playing game. Not as flexible as a tabletop setting, but still with a certain amount of freedom and agency for the player.
And saying the player's avatar does not, cannot, and should not survive is one of the worst sins you can commit in a role-playing game. Railroading of the worst kind.
As a writer, I find the existence of a "Happy Ending" mod to be, frankly, offensive. Did I love the ending to the Mass Effect series? No, not really. I didn't hate it, but the fact that Shepard (and possibly others) dies has never been an issue with me. That was the ending to Shepard's story, just as it was always meant to be.
That's your opinion, and that's fine, because I find the endings as written to be kinda offensive myself. Saying this is how Shepard's story "has" to end is the antithesis of role-playing. Saying my character has to die, and worse die doing something I find wholly repugnant, that it was "meant" to be is, to me, detestable. This was not supposed to be an on-rails shooter where the player just has to point the gun and enjoy the story being told them. It was supposed to be so much more.
What this mod does is provide an option. And fortunately options are...optional.