There's actually one major difference between SACD and Blu-ray (well two actually). SACD was not a recordable format, and even if it was, there was no source of audio you could record which would take advantage of the increased audio resolution.
You can already get Blu-ray recorders, and with HDTV you already have a source which is worthwhile recording on Blu-ray over normal DVD. In addition, Blu-ray has potential for computer data storage - again especially with recordable Blu-ray discs (but is already being used to some extent on the PS3 - sadly the drive on the PS3 is a little to slow, thus the tendency for data to be copied to the PS3 HDD).
It's not correct to state Blu-ray is going to be the next SACD. However, I'm sure its not correct to say Blu-ray is going to be the next DVD. DVDs success was unpredicted and unprecidented. Many factors helped DVD domiate - not just convenience.
Remember, before DVD (in the US at least), you couldn't buy big name, theatrical releases months after their cinema showings for a decent price. Most of the VHS tapes were priced for rental - typically you would have to wait longer for priced to own VHS releaes. DVD not only offered audio/video and playback enhancement, but it enabled people to "own" recent Hollywood movies for the first time at a sensible price.
I don't really see what most of the complaints are with Blu-ray. The fact all players play DVDs mean that unlike Laserdisc or VHS, nobody HAS to repurchase any title. The fact all players upscale DVDs to some extent (some better then others) shows that the manufacturers are even making your DVDs look better than ever before. Disney, and other studios likewise aren't going to drop DVD anytime soon - check out the Digital Bits for some facts (they regularly update the following information at the foot of their hompage):
SOFTWARE MARKET SHARE - BD VS DVD (YTD through 11/16/08 - Nielsen/VideoScan):
Week End. 11/16 Blu-ray Disc: 4 DVD: 96
TOTAL U.S. PLAYER SALES (last data reported 8/24/07 - CEA): 133,443,356
Editor's Note: The Consumer Electronics Association has discontinued its reporting of DVD player sales data, and they have not begun reporting on Blu-ray player sales. However, our original CEA DVD Player Sales chart will remain available here for future reference.
They no longer count DVD sales, but at last count there was over 133m DVD players sold in the US. Do you really think Disney's going to ignore more than 120m players anytime soon?
It's a shame that it appears some DVDs are getting lesser features than Blu-ray discs... but I'm not convinced this wouldn't have happened anyway. Did Blu-ray affect the decision to make Cars a single disc? Is Blu-ray responsible for the Enchanted DVD being so poor? Or did Blu-ray result in POTC: At World's End DVD release from being lighter than the other Pirates films? Going further back did Blu-ray stop the 2 Disc Vault Disney titles?
I think studios had learned that they didn't need to go all-out to sell their DVDs, they could sell as many without going that extra mile. So I think its something that would have happened to some extent regardless of Blu-ray or any other format. But at the moment they
do have to go the extra mile to make the Blu-ray seem attractive, especially given the pricing premium charged for Blu-ray (but I repeat, there's lots of bargain Blu-ray discs if you take the effort to shop around).
So to conclude: DVD isn't going anywhere fast, Blu-ray isn't taking over anytime soon, when it does, you can still play your DVDs and they'll look better than ever so the option to upgrade is not being forced on you and finally Disney were cutting back on supplements before Blu-ray was even launched.