The point is that languages don't fall out of favor among hackers simply because they become popular. They fall out of favor because hackers are very promiscuous about their programming languages and move on to something better when they find it. And since there is a whole lot of creativity in that community, something better comes along pretty regularly.
I don't expect innovation to continue at this pace forever, though. Many of the good ideas making their way into the mainstream today were invented in academia 30 or 60 years ago. Eventually the popular languages are going to converge around a more-or-less standard set of features, and there will be less need for such rapid innovation.
Encouragingly, I think we're catching a glimpse of one area around which programming languages are going to converge in the near future: pluggable type systems (that is, optional type systems which are dynamic in nature and can be removed, modified, or extended). This is happening from both directions: dynamic languages are adding optional static features, and static languages are becoming more dynamic. Consider the signs:
- Python 3 will have function annotations, which have no semantic meaning on their own but can be interpreted by 3rd-party libraries.
- One of the JRuby developers is working on a toy Ruby dialect that adds type annotation to the language. Interestingly, the type annotations are valid Ruby and would just be ignored (or immediately garbage-collected) by a normal Ruby interpreter - giving them a pluggable flavor.
- The next version of c# will support dynamic method lookup.
- Qi, an interesting lisp-like language, has an optional type system.
- Boo, a statically-typed scripting language for the CLR, has a "duck" type which allows method lookups to be done at runtime.