I am not a fan of this proposal at all - be it from MS or Sony. I buy into a console generation for 3 reasons:
(1) Uniformity - everyone owns the exact same hardware and it is a level playing field online and offline. The market is not split, and developers can focus their attention on making ambitious games for one hardware spec.
(2) Optimization - as the generation goes on, developers learn to optimize and make better and better use of the static hardware. For example, games released at the beginning of PS3 and toward the end of its life cycle were very different.
(3) Longevity - a one time hardware investment and being able to use the said hardware for at least 6-7 years. Unless the system breaks, the only investments are accessories and games.
I would rather have Sony release PS5 by 2018 (5 year console cycle, major hardware upgrade), than a mid-gen refresh that splits the PS4 market and forces developers to split resources in optimizing the same game for two different PS4's. The simplicity of owing a console would lose its charm, at least for me.