The key thing about an extension language is NOT it's adaptability over time. The key thing is that it works as a tool for the people who might want to write extensions to applications. As a general rule, the extension developers won't be the full-time developers of the applications. Instead, they're going to be the slightly-more-technically-adept users of the applications.
If you want extensions to be developed, then you need to remove barriers to entry for those people. And probably the most important barrier is time. And it takes time to learn a new language sufficiently well to do useful things with it. Therefore, the sensible choice is to pick a language that is broadly known by the sorts of people who would be likely to want to develop an extension. The next best thing is to pick a language that is really easy for those people to learn (by virtue of being similar to other languages that they probably already know).
Guile fails on both accounts. It may be a fantastically great language, but it lacks the widespread use necessary to be a successful application extension language.