From an engineering perspective I don't see how they could make easy, modular GPU or CPU upgrades, which are really the only things that would give noticeable boosts in performance. It's not easy as plugging in a expansion pack with an extra 4 MB of RAM, ala N64. And there's no way they would make it as technical as PC upgrades, as console gamers don't want that and it would also kill any reason to get a console.
Plus, I think Sony and Microsoft like the half-step upgrade thing a little too much now to do anything else (basically a freebie of a "new console" but without having to go all-in on actually designing a new one).