Jack is solving a different problem domain. There is very very significant differences and requirements between people wanting to do real time audio editing vs multimedia playback.
Plus it's possible to play regular applications through PA and into Jack, if you want to use a familiar application or media source.
> Still, a good start would be a aggressive purge of legacy solutions - arts, esd, mas, various parts of libalsa.
Ya. Unfortunately that requires rewriting applications that depend heavily on those API.
For very modern-style applications that use things like Phonon or Gstreamer, this shouldn't be a big problem since those things are designed to be modular. Arts is being taken care of through the KDE folks, who are forced to rewrite their applications to work with QT4 anyways. Mas isn't popular. PA is compatible with Esd.
Then there is OpenAL, SDL, and numerous other much more minor sound APIs.