One thing I always liked about D&D was that in each new addition you can count on is the DM Handbook, the Player's Handbook and the Monster Manual.
In fairness, SR has a similar setup...core + magic + weapons + matrix + options + rigger...wish that "critters" was also in there.
Plus, since so much of the game is the setting itself, you can still make use of old sourcebooks like Vice and Feral Cities from 4e (and from older editions, I just don't know any).
And this is yet another reason that the largest majority of "fluff" should be in separate sources. This way, those sources stay good throughout the life of the game even beyond the edition it's published during.
As to the sales issue, these sources can be reprinted as editions go on with updates on the various pieces of information, possibly even opening up 'special' compilations of several of these sources. Lord knows that many of the older setting-only books need updated versions, and this tendency of overloading rules supplements with fluffy bits is just, IMO, creating an excuse not to redo those.