This will be a long post...
As for why I say the manga has gone down hill is because of 1 obvious reason imo, and that is poor planning. It all started when Madara was summoned as an Edo. While that was incredible at the time, for the future it made things complicated due to him and Obito sharing the same goal/role in the manga. I don't think Kishi planned that far ahead. Madara as an Edo casually destroyed the 5 Kages so at that point there was no one stronger than him individually, while the second strongest was Obito. These two were already far above everyone else so Kishi introduced Hashi, Tobirama, Minato, and Hiruzen to help stop Obito, Madara, and the Juubi. That was overkill in my eyes favoring the Alliance so Kishi had to give someone a power up but he gave it to the wrong person...This is where things went downhill fast. Once Obito was Juubi Jin and we seen how strong he was then it was obvious Madara would be far stronger once he became the Jin. The only way for Kishi to alleviate this problem was to give Naruto and Sasuke power ups, but in doing so he showed us someone potentially stronger than Juubi Madara and that was Kaguya. Once Kaguya was introduced it threw things even more out of loop and the BS grew. Due to the poor planning of Kishi he made Obito a Juubi Jin and Kaguya final villain, both things that hurt the story. The only Juubi Jin and final villain should have been Madara. That would have prevented a lot of the mangas down falls in my opinion.