I find it rather ironic that decade's battle ended in the world that it began in. It felt like a full circle had happened when they returned to Natsumi's world. It was also interesting to note that the Rider that proved to be the biggest help had the least to gain from the entire battle. W's world wasn't on the edge of collapse, in fact he didn't know Decade outside of a passing glance and yet it's through his help that the destruction of the rider worlds end.
