Those strata graphs don't prove anything about stability, because a small number of lines of code (that don't make any difference to the graph) can wreck stability. Whereas a complete rewrite of something quite large can be utterly backwards compatible (bug for bug, even) and not break anyone.

Too much horizontal striping in this kind of graph can be bad news: the project isn't improving its old code. That could be because the code is perfect and doesn't need anything, or it could be that the maintainers are not confident enough to touch it.