I find it sad that the concept of the narrative arc is completely lost on you.
If you want to start getting all quasi mystical, the arrival is no less a part of the journey. It's why the journey is begun in the first place.
Yes and no. Where we are going is important, the ending perhaps is the most important part of any story, because it is the story teller's last interaction with the audience. The lasting impression, the emparted moral, the conclusion of loose ends, the resolution of conflicts, this is the time to attend to such things and they should all be attended to in an order that seems a natural progression of the narrative from what came before.
If we truly are rating a story collectively then we can't so casually dismiss the ending. It puts everything that came before into context and is supposed to obey the rules that we have established prior to that point. To say it doesn't matter how bad an ending is isn't rating collectively, it's rating selectively. You are discounting what you don't like as opposed to taking the story in as a whole.
As a personal aside, It is very odd to me that you separate the end of a story from the rest of the story as two district and non related entities. There seems to be a fundamental device between people that do and people that don't.