SACD's biggest threat is to vinyl.
For a little over $1,000, one can purchase a SACD player that approaches the sound of $10K vinyl systems (TT, pre-preamp, cartridge). SACD gives 3 times the playing time, no pops, no need to flip, can use in your car, multichannel, etc.
Vinyl will not die overnight. I'm not going to get rid of my vinyl, but may never buy another record. My two boys will never buy vinyl.
The CD with its circa 1980 technology will last for at least another 10 years, but will be attacked from all fronts: MP3, Windows Media, SACD, DVD-V, DVD-A, and mini-discs to mention the major ones. It's popularity is on decline.
CD supporters remind me of DOS supporters. "Why should I buy Windows? It costs more, I have all this DOS software, and I really can't see a big improvement. It's only Microsoft trying to make more money."
The CD will decline in popularity--every format does. Each of us needs to decide when and which new format to hitch to. Many people will wait until the last minute. Many people are still using DOS and OS/2.