I think it was all because of a misfortune, After writing chapter 145, Amazume, Ryuta the author of this manga suffered brain damage as a result of a serious accident. But he still wanted to finish his art and decided to take a piece of paper and shit on it then give that paper to his publisher as a final arc.
when you think about it, there can not be any other explanation for finishing such a wonderful manga with an ending like that.
After reading chapter 145 i was planning to give that manga 10/10. it wasn't flawless but it was good enough for me to be a masterpiece. and a few minute later while i was reading chapter 151 i burst into tears because of anger. I can't find the right words to describe my disappointment. I feel betrayed and I really feel sick when I think about all these good times in previous chapters.
So, long story short, F..k you Amazume, Ryuta. This is my final words for your bullshit ending, F..k you!