Backwards compatibility isn't just some small chunk of software that reads the disc correctly. In order to play previous games, you need to either include most of the original hardware or have software emulation of it. Allowing this on the previous generation wasn't too difficult, as the generation prior was very simplistic. Now we are at a point where including the demands of both the current/upcoming and previous generation of consoles into a single unit is a tremendous hurdle to overcome, making it it not cost effective. Beyond that, I would say that the idea of trading in one console for the next is mostly a thing of the past. With the rapid proliferation of gaming we've seen this past generation, where almost every home has a console, trade in value is so low on consoles that the developers should expect that most consumers will either keep their previous generation consoles or be able to find one for such a cheap price that the added cost of R&D for the developer and unit cost to the consumers for backwards compatibility is just not manageable. If you could buy a used Xbox 360 for ~$75, why would you want to pay an additional $100 for the added development costs of backwards compatibility.