Have you
really gone trough HF, Jsmith? Because that is the route where Shirou's mentality is the strongest. The only route were he really knows what he truly wants to do. And that is to protect the ones that are close to him.
And why isn't the ending believable?
And Saber is not the main focus of F/SN she never was, she is the poster girl though.
And in UBW she isnt even close to being the focus
She isn't even the third, that place goes to Rin, I wouldn't even say that she is fourth since the route is more focused on other characters dilemmas than Saber.
Saber had her time to shine in Fate and got the ending that was best for her. her wish was never meant to be granted, and she realized that herself. Had Shirou or anyone else wished for her to stay,and the grail had granted that, she would have been corrupted.
And the manga will without a doubt end up the same way as fate did in the end. The mangaka has always been looking forward to drawing the last scene, he said so in the first volume. He won't change that.
I don't even understand why you had such a problem with the ending. Of course it was sad but thats life. Saber was doomed to disappear.
I always found her fate in the UBW good ending rather sad since she never found her 'answer' which means that she still doesn't know what to do, what her place in the world is and so on.