I don't think that's particularly relevant, or that many people would care. The bogies are only ten years old, the traction equipment is new, the bodies are aluminium and therefore corrosion-resistant, the interiors were given an extensive refurbishment only a few years ago, and the things were built like tanks. They represent a pretty good improvement over what some lines are operating with. Agreed, it's not suitable for everything, and it's ugly as hell - the D Stock was always quite boxy-looking - but it's a decent and flexible bit of kit at the end of the day. Ultimately, the fact that it's D Stock doesn't really matter. It's what happened to be available. The concept, and the technology it has spawned, is ultimately what matters.