Stanhifi,
"8 tracks, beta, audio cassettes, VHS"
they've been extinct for a long time but they can still be purchased. You can get 8 tracks on the internet or at second hand stores on the cheap, audio cassettes are still in some record stores, vhs is still in stores although not the main video format anymore, and beta is still used in cameras and can be purchased on the internet.
So, I guess your point is that SACD's will be available for a long time?