I'm no expert but we are talking about console OS here, not for pcs and a variety of hw configurations. They only need the os to allow gaming and accessing to controlled apps simultaneously, and considering how vita handles that with a fraction of the ram ps4 seems to have, i doubt sony will fail. Even smartphones do that today. How many apps will people be using simultaneously? Browser, netflix, youtube, twitter, screen capture, music app? I don't see what 360 does better than ps3 besides party chat( considered since day one unlike sony who had to catch up while reducing os footprint) in similar conditions. If ms were os wizards as some say , there would be huge differences.
Maybe Durango's ram is for using office while gaming who knows, but i'd say with smartphones just getting 1080p displays, and most apps will be the same with much more ram available... i see no problem at all , unless ms comes with something unexpected.
Maybe the flash hdd could be used to store the game when pausing, instead of main ram? Or still too slow?