Well seeing as the fundamental parts of both the ps4 and xbox one are basically a pc with a fixed set of hardware wouldn't making them upgradeable just make them into a pc with a custom os...
I do like the idea of a more modular upgrade path to consoles, think making them into 4 modules, processor, memory, graphics and storage so that upgrades could be released by the manufacturer at key stages in development. It does however cause issues for dev's and ensuring everyone has good performance etc.