Dejen's last lap may be faster/more impressive because, maybe he runs a faster 400m than KB. That doesn't mean he was capable of running 5 seconds faster than KB over 5000m, and almost 15 seconds faster than the fastest time he (DG) has ever run in his life.
I do think Dejen should've beaten Mo in London. And I suspect Dejen felt the same way ... He didn't look happy coming in for a silver medal. I think he simply misjudged his race, and his last lap. He thought he could catch and pass Mo, but simply put himself in a bad position to do so.
Komen, for example, never had a particularly last lap. So comparing his last lap if, say, 57 or 58, with KB's or DG's is somewhat silly. He just didn't run that way.