I think the point is not that endings are generally and extrinsically sad, but rather that by definition, an ending is a thing which is sad, if we take the existence of such a thing to be good. (The ending of a bad thing, for example, is an exception, though generally because it allows for the existence of good things). The response, then, would not to be to try to improve endings, but rather to try to do away with them (and, barring that, improve the extrinsic qualities of the non-ending parts).