Consoles have to stay fully compatible with the entire line of games past, present and future. You can't expect them to upgrade, else they run the risk of splitting their games lineup into games that work on the new hardware and games that don't. Noone wants to see 'minimum specifications' notices on console games; compatipility is a massive driver in the choice for console gaming, after all.
Really, the only way I imagine you could expect to see upgrades is if they also offerred them as user-installable bolt-ons to the existing models. Design/development of those would surely negate any perceived cost saving from the new model.
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