So you're saying that porting once every 7 (!!) years, is more painful than writing and maintaining the abstraction layer?
GStreamer 0.10.0 came out in 2005. It's parallel installable with older and newer versions. During that time, KDE has been through many 3.x and 4.x releases, and had their own share of porting work. I find it difficult to understand how exactly you'd be "stuck with GStreamer as of two API/ABI versions ago", and what you gained by using Phonon. And while I admit I'm not very up-to-date with happenings in the KDE/QT world, isn't Phonon going to be replaced with QtMultimedia and QtMobility?