Swordsinger is an immensely useful class in higher level parties. Sadly it does not really shine at lower levels, and I guess (never tried it myself) the class is a little bit boring to play, since you do not really have that many fun skills to play around with. BD outshines SwS a bit, since offense>defense in standard pve. But as soon as things get a bit more tricky, mp cost reduction / skill reuse timer songs, protection songs and so on really shine. And having song of hunter never hurts.
SwS is far from a dying class, I know quite a lot of people who sub it because it's a wanted class.
Sure, song of hunter is a useful buff. Its one buff though. Compared to that, BD's have several of equal usefulness, AND BD's are useful to nukers and physical dd's before level 77. And since damage > protection in most aspects of L2....
Swordsingers... extinct? That is the [deleted] thing I've read in official in a long time.
I, among many others would take them over a BD if we had a choice of only one over the other. You cannot imagine the kind of advantages a party with wind, warding, invocation, vitality, hunter, earth, water, and/or etc has over a party without.