@Lyanhyrt
I feel like it's a common emotion when reading long running manga, especially Shounen. On the top of my head series where something like this happened are Tokyo[N O T S W A S T I K A]Revengers, Yuna's onsen water, Tsugumomo (though that ones a rollercoaster and doesn't seem to be ending anytime soon), The 5-tuplet harem, Hatsukoi zombie, Boku no Pico Harem Acedemia, Infection, Murenase Shiiton Gakuen(another roller coaster, though recently it has more downs than ups), Beastiality-stars, Thus Killing Bites, aaaand many more... There are exceptions of course and sometimes a series can bounce back, but looking at how this series is ending it's unfortunately gonna end with a whimper so it seems. It can't be helped when an author starts an underwhelming arc and either has to rush it or go with the original plan and piss off the readers for the remainder of the arc. It's the curse of a long running manga i tell ya!