In terms of modularity, I could see things becoming like 2nd edition to an extent. Looking through the Player's Handbook and Dungeon Master's Guide, there were a lot of optional rules, such as weapon modifiers based on armor, critical hits, story-based XP, death from massive damage, material components for spells, and proficiencies, which was an entirely optional chapter.
The problem was that many of the optional rules stopped being optional if you used any supporting modules or supplements. Most of them assumed the use of many optional parts of the core books, especially nonweapon proficiencies.
As with most things involved with 5th edition, I don't think the core books will be a problem. The real test will be in how WotC supports the game beyond that.