It wasn't even the concept of aliens that did it for me, I thought that was some clever world building. What the final straw was for me, was the piss poor execution of the whole thing. The manga built up Madara as a powerful presence early on in the series, then continued building him up as the final villain and even managed to make the character live up to the ridiculous hype (a rare feat indeed)... And then he was just tossed aside. Years of hype. Years of buildup. Gone. And as quickly as he was tossed aside, out of nowhere we were introduced to the FINAL final villain. It was a total joke.
Beyond that though I still think Kaguya in general was poorly handled. The manga was doing great dropping these periodic teases to the world-building. And then it had to blow it's load by not only showing us one of the few characters shrouded in mystery, but had to spell out every single detail about these mysterious characters. A little mystery goes a long way with something like this, and certainly would have kept the series alive far longer through constant theories/discussions if the manga had simply dripped little fragments and hints of what the history of the world was and left it to the audience to piece it together.
I like how the story went, Madara was powerfull enough to be unbeatable, but he became naive when he discovered that he was the only one that knew what was on the tablet.He awaken MS and EMS via what was said on the tablet, so he believed the tablet and keep this knowledge for himself, 2 mistakes.Years of build up to attain this point where it was revealed that Madara was manipulated his whole life by words on a tablet, I liked when Madara understood that he has always been a little tool in BZ hands, that must be hard for an arrogant man like him.I like when arrogant men understand they were little shits.
Kaguya was introduced 60 chapters before the end of part 2, and she wasn' t the final villain because it was said that she feared something.Kaguya was just an introduction for part 3 villains, and that' s a big mystery.
We don' t see theories anymore because we didn' t have new content.There are lots of theories to be made in relation to the information we get in Boruto 14, why do Ootsutsuki clan collect fruits for exemple...