The kind of knowledge that gets obsolete is replaced by stuff that's very similar.

Doesn't matter if it is a new language, framework or whatnot. There have been very few advances in our field, stuff changes incrementally. So you have seen everything before, it just has a different twist. You can pick this crap up in a fraction of time, as the concepts are already internalized.

... Unless, that is, if you used to call yourself "X developer", for whatever value of X. Then you are unidimensional, and screwed.