I have a couple of problems with your explanation.

First, the performance and rapid development gains in tight coupling typically aren't as significant (and, in the case of rapid development, perhaps aren't even typically extant) as they are for simply choosing the right tool (language) for the job. The analogy you provide doesn't strike me as being particularly viable.

Second, the importance of incremental performance gains from tight coupling decreases over time as system performance capability increases. The reason that UNIX was able to overcome less modular designs didn't have so much to do with the fact that less modular designs were superseded as the fact that less modular designs were no longer a simple necessity of hardware restrictions. I tend to think that as system performance capability increases, we'll see further evolution toward modularization, and what we think of as loose coupling today may be classed as tight coupling in a few years.

Older, less modular OS designs didn't develop so much because they provided a simpler path toward development as because A) greater modularity in design hadn't really been experimented with very much yet and B) more modular design simply wouldn't run on the comparatively limited hardware available at the time.