I don't think that "core" 5e will look anything like "core" 3e or "core" 4e. According to the quotes we've seen from the developers so far, core 5e is going to be very small and lightweight. It might actually only be a section in a larger main rulebook that's chalk-full of modular, optional rules. Anything that isn't absolutely essential to the D&D experience (like character race) is likely to be moved out of the 5e core rules entirely and placed in an optional rules section.
I agree. With core reduced to only the most basic stuff, the 5e PHB would be far to small and no one other then 1e fans would buy it until alot of modals were released assuming interest didn't disappear.
Plus his comment on the warfare option being in the same DMG as "core" makes me think the PHB will be designed the same.
I don't even like calling it core as such, maybe basic would be better.
So in the PHB, MM, and DMG you'll start off with a section on the basics, basic rules, classes, races, spells (spells may be in a seperate section devoted to all modals containing powers), Then you'll have different chapters being devoted to modals, and each modals can be combined in different ways. Some moduals may even add or subtract from aspects of basic.
In fact taking a race and class maybe a rule in basic, but the actual races and classes maybe in a seperate section.
So basic section might be just basic rules like the first section of the phb 4e, where they explain stats and how to build a character along with basic rules. It would tell you to pick a class and race, but it wouldn't choose them for you.
The next section would be picking optional modals on how to build your character. One modal might cover race as class via multiclassing, another regular multiclassing, another themes, another kits/subclasses, another prestiges classes and paragon paths, another power sources, and other optional choices. Then when the rules modals are done and you've pick your modals comes actual content, classes, kits, paragon paths, spells, items, with each modal telling which content section and or specific content is apprioate or needed in the second half.
Example
Pick a class and race. Here are the basic rules for how classes work, modals may adjust this see modals section for details, you most pick a class, classes are blah blah, have hitpoints, blah blah... See the classes section to pick a class after you've choosen our character design modals. Maybe with suggestions on the most basic classes.