>does every manga that ends in 2020 always ends in disappointment.?
No, it's like 95% or more of manga in general. Big part of it is serialization format.
Author comes up with a premise, gets greenlit and starts publishing. Doesn't mean they have enough stuff to keep going. Very often they don't, and required consistent output might make it worse. Can't take a break to figure stuff out.
And then there is popular stuff that just keeps running because it's popular, rather than telling a contained story. And then, when it milked dry and ratings fall, it gets "finished" or just axed.
So much manga get ruined by those things.