Cohesion and coupling are still used in software design. But I'll disagree with Plane Sailing that early D&D comes out looking quite that good in comparison to later D&D, on those terms.
Take 1E, for example. It is lightly-coupled in one respect. You can drop and add spells, pull out some of the optional rules, etc. The whole thing will more or less hang together. On the other hand, you do have effects that have serious consequences. Take away appropriate magic items, and watch the party run away from more and more monsters. We don't typically think of that as a coupling issue, because we see the effect so easily. One might easily respond to that argument, "Well, if you unplug the toaster, it won't work, either." That's true, but also doesn't change the fact that if your power goes out, you won't be toasting bread. (And things like generators that get around this are their own complications, and only made possible because the power supply in a house is a lot more cohesive than any roleplaying system.)
Or in software terms, early D&D is lightly coupled, but it is not robust. Or rather, is is robust in select (and very useful) ways, but not in others that people have nevertheless tried. later D&D is more ambitious, and thus the modules and coupling between them involves tradeoffs in order to stay robust over a wider scope of activity.
It's like a folder with paper versus a word processing program. Those of you as old as me probably remember how long it took MS Word, Word Perfect, etc. to become as reliable as a typewriter, never mind a notebook and pencil. (Net reliable, over the scope of work.) Like 1E, paper in a spiral notebook is modular and loosely coupled (literally), and robust in its own way. You can take it on the bus, and it doesn't matter if you you lose your internet connection, for example. All the parts are readily understandable to everyone. If you take the sixth page out and move it to the back, the consequences, if any, will probably be obvious to you. But backups are difficult, search is tedius, and so on. It doesn't easily scale to handle many issues.
Later D&D, in contrast, is more limited in one respect, like a MS Word 2007. If you don't use styles the way it expects, it will actively fight you as documents get more complex. You are dependent on outside things that most people don't understand fully, such as having a working computer on which to run it. The program, not D&D. But note already how being able to use a program with 4E is curbing the ability to use 4E without it, even though 4E is perfectly playable without any software whatsoever. And let's not even get into how spell check programs turn people into worse editors than they otherwise would be. Yet note how much more easily it was to turn base 4E into Dark Sun 4E, than earlier. The system was robust in the face expected changes within its more ambitious scope.
Pencils are simple, but very complex (in the OP's terms). There is not a single person on the planet that can make a #2 pencil, from start to finish. Yet the "user interface" on a pencil is perfected. Part of the problem with 21st century D&D is that the user interface has not caught up with the ambitious scope. The coupling and design of D&D is way ahead of the presentation. And really, it always has been. It is merely that with earlier versions, the scope was narrow enough that one could encompass the design, warts and all, relatively easily.
