Oh wow, an event that was obvious it was gonna happen since chapter one actually happened. Who would have thought.... I mean, it's not like we have hundreds of manga with the same type of development.
Not having what you consider satisfying payoffs doesn't make a bad manga. There's nothing right about what you're saying. It's literally just you being entitled.
My dude, you quite literally said "If these dragons don't suffer in their last moments than this manga is garbage plain and simple." This, very obviously, is you expressing your desire for the story to go in a certain way and saying if it doesn't it's garbage. Get off your fucking high horse...