I want to share my thoughts on this:
First of all, I expect both this rumor and the mentioned 3gb of Xbone to be the first step ram-usage. Both systems are going to be a lot more extended to what we know today.
The rumors had MS always to "limit" ram to 5gb, since ever. This was because they knew they would have an advantage over Sony in terms of size. Looking at what pc-games need nowadays this was a "safe" barrier for games and would still be enough and still leaves room for optimizations.
PS4 os-size was at 512 mb when they had "only" 4gb of ram. So MS could easily play safe. This was a moment when Sony was still developing their own gui on top of bsd, because (as rumoured) they wanted to build there own gui, not relying on available stuff (read: no X-server).
Then Sony showed the world that they had listened to devs and implemented 8 gb of ram. I here feel for the devs who build the os because Sony wanted to do a lot of stuff with the os and they were limited to 512 mb which is really low given what they wanted to do. Also keep in mind that you have to do a lot of development for building a custom gui on a given os.
I had several talks on this forum back then (I remember especially arguing with VGKing) where I said that I always believed in an advantage that MS has over Sony because they know how to build OSs. Look at 360 and PS3 and you know what I mean.
After the 8 gb reveal there were almost instantly talks of 1 gb ram for Sony's os. 512 mb was too low since the beginning for sure.
I think even by now Sony is trying to make a decent OS on top of bsd while integrating social stuff and making it possible to run apps in a closed environment.
It could be that the os devs were really happy they don't have limits now and Sony told the guys "ok, we will play safe as MS does. We won't limit games by now and we can further optimize in future".
Limiting ram for devs while making room for individual apps and integrating social stuff and making your own gui-approach requires a lot of work and time, even for experienced os-devs. The kernel of bsd has low memory-footprint but operating an os is way more than just being able to run a kernel.
So I think it is no problem to "limit" ram to 5gb right now. For both consoles. Let them optimize them and let them see how ram-demanding future apps will be.
