Well, I'm assuming that anyone with the technical chops to assemble a PC isn't going to be representative of the average user. I'm actually planning a long-term project at my home to make everything controllable through a "house computer". Basically, when all is said and done, I plan to have some sort of interface in each room that allows me to control music, television, temperature, lighting, etc throughout the house. And, taking on tasks like that require going above and beyond what the average user requires or wants. That sort of capability will still be available to us.
With a more "ala carte" OS functionality, you'll still be able to assemble the monolith if that's what you choose to do. And, for some people that may make sense for an indefinite period of time. Just going from my experience dealing with computer users, the large OS has been standard, but it's really overkill for the vast majority of them, and it's becoming moreso with "package" software becoming decreasingly relevant (remember some years ago when you might buy a program like "mah-jong" as an executable on several floppies
).