I certainly think things like "No Silver Bullet" are just as applicable today as they were decades ago, especially as we see more and more young people come into industry and think x is the latest and greatest killer language/technology and all the other technologies will die because of it.
Granted, the references to Ada or sharing computers are antiquated, but the concept of accidental and essential difficulties, buy vs. build, how code is complex by definition because we don't repeat parts, and all the other theoretical topics are still completely accurate and relevant.
The other argument for why TMMM is relevant is that, it's not really about software itself but about how programmers get things done. In this way, it's hard for it to become obsolete.