I agree, the IDEA is good, the execution absolutely piss poor. Why couldn't the Starkids be a faction of people who believe if you crack open the multiverse you'll destroy everything. Making it seem like doing your mission would make you the baddie, but come to find out the leader of the faction has already been to multiple universes and each time he goes he gains power (perhaps limiting the Star powers to one per universe), and he doesn't want other people discovering this source of power until he gets as much for himself (themself) as possible. Thus motivating you to not only persue the main quest but to also do NG+ and keep growing your power base.
I don't know. Why doesn't Lord Of The Rings end with a massive, climactic fight between Gandalf and Sauron, Gandalf obliterating Sauron with a blazing spell of destruction? Surely that would be cooler than just dropping a ring into a volcano.
There's no useful answer to your objection. You don't have to like their decision, but it was their artistic creation and it's perfectly valid.
The narrative throughout - The Pilgrim, The Emissary, The Hunter - are people who realise they can go through multiple universes and have to come to terms with it and find meaning in it all. This isn't a narrative setup that requires anyone to "save the universe", and indeed needing to "save the universe" would ruin that narrative. One might note that narrative also supports the player just sodding off and ignoring the main plot. Find your own way. Like Achilles was offered the choice of a short and glorious life, or to live long as a farmer and be forgotten. Well, maybe in another world, Achilles chooses the farm.
The problem with this idea is that in other for there to be something to explore, or at least something to find, you have to accept someone was there before you. Discovering an ancient temple means someone has already been there by sheer requirement.
So you'd say Christopher Columbus discovering the Americas was no big deal, because the native Americans got there first? It wasn't totally amazing for someone to find the tomb of Tutankhamun, because they were just finding what the ancient Egyptions buried?
A game where there isn't any conflict, or have factions, or any BBG could work in theory. But there would have to be some other core element of player agency. Player agency is what makes a game a game, without it you just have digital space. So perhaps something like early Minecraft where there weren't enemies, but you have agency to mine blocks and build yoursefl....something. I don't see why that couldn't be the case in a space exploration game. You go around with a ship from planet to planet to not only get resources but also find a suitable place to build a home, upgrade your ship, and maybe even expand out with your own little alien colony.
I just don't think the concept works in a AAA-galactic wide RPG.
I don't think this argument really holds water. This is a bit like arguing you can have an action game requiring fast thinking, reflexes, etc. and you can have a strategy game where you need to plan, think, devise tactics and so on, but it doesn't really work if you mash them together.
And yet real time strategy games were, and remain, wildly successful.
I would argue that there is an incredibly obvious appeal to a sort of simulated whole world, where countless people are going about their lives that you can interact with in all sorts of ways. As you note with Minecraft: people can make their own amusement farming, decorating their houses, etc. The Sims, and so on. There's an obvious appeal to a narrative adventure full of quests to perform. It seems to me eminently sensible to have the two merged: a narrative adventure built into a highly developed, wider setting where everyone is going about their daily lives. I also think this idea of stuff going on the players can explore but may be off-piste from the main quest is absolutely consistent with the idea of traditional tabletop RPGs: it's part of the immersion of being in a world.