Good to hear. Games technically will continue to improve.

so does this mean OS will be slower in future...?

No. 

so why they reserve it in the first place?

Awesome analogy, thanks for that :)

Maybe Sony was afraid of Kinect and/or had plans for the stereo camera to match that. Maybe reserved for VR yet that gets an external processor with the headset. Always better to have some overhead than to risk another headache over getting the xmb to work in game like last gen and not be able to add certain features like party chat.

Just being over cautious with their first dev kits to make sure everyone was creating games for the very minimum of what the console would have left over when the UI was all up and running, with the intention that if the UI ended up eating more resources than they anticipated then that wouldn't effect devs who were already mid production of games for the system, More time for Sony to streamline down the UI and to optimize a few bits in it will shrink the resources that needs in the background.

Similar things happened with other systems after launch, only exception where a console lost a core was when MS stripped down the amount of hardware that was included inside Project Natal since the original versions of that kit had a CPU inside then Kinect sensor so it would add on the processing power it required to handle kinect features with no impact on the 360s performance, but during developement that seemed like it would end up being too costly so instead the Kinect ended up harvesting 1 of the 3 cores of the 360 for it's functions for Kinect based games (tis why Kinect games tend to look less.... gears of War'y, generally)

All this OS talk. Memory is more important to the OS and its smoothness or speed than gwoevee many cores are reserved for it. 

The "Background" tasks handled by the OS are usually very basic tasks. most conokex of which is probably in game music, party chat/cross chat.... etc. But that's basically it. Proof is that there isn't part of the GPU being reserved for the OS  When you leave the game, and go back to the OS, the "background" core/hypervisor just switches task priority from app memory (5GB) to system memory (3GB). so basically, All other CPU cores and all of the GPU suspends whatever jobs they were doing and switches to resuming whatever jobs were previously suspended on the OS side of things. its kinda how smartphones manage their processing resources. 

The only thing that can really noticeably affect OS performance is how much memory is being reserved for it. As long as most of the OS is stored in system ram it will be super snappy. If system ram is OS allotment is dropped then parts of the OS would be cached to the HDD (ps3) and shuffling the data to and fro will cause hiccups. 

That's the thing with consoles a few years in and they start to optimise the hardware... Efficiency, same hardware a little more juice!

Huh. Ms did this months ago if my memory serves me. Hopefully third party devs can now create games for the consoles with 7 cores in mind.