Does anyone think most manga writers in general lose track of main plot of the series the more they prolong it. From what I seen this is very common in long running series. They always throw in some random plot to make the series run longer even when they don't really make sense with the rest of the story. Then later to compensate with this change they do stuff like change a character's character which is very annoying and clear indication of a bad writer.

Well, mangaka aren't allowed to simply quit their stories, especially if their stories are long and popular. In Japan, the idea behind manga is that the writer should continue to produce their story for as long as they can, so that everyone can enjoy them longer. It's pretty impressive when a manga artist has made their work so long, and that their work is so popular, that they are allowed to choose when and how they end it.

I have no issue with making them long as long as logic and common sense is applied. Problem with most is they throw in a random plot that makes no sense what so ever with the rest of the story instead of building it up. Their are few that actually built up to the plot twist but they lost track of their main plot or changed a character's character to compensate for it.