I disagree with it. Look at the ps2 singstar series - old discs didn't work themselves, but You could run their content with a ps3 disc. All we need on new playstation is a new Rock Band game and it (content transfer) can be done even with download only by now. I would rather worry if M$ will make old instruments work with the new console, because of their controller policy. No worries for the rest, when all the wii's and ps3's gear is USB and that means universal.
As for dlc being compatible on the next consoles... If you're playing on Ps3 I'd say the chance of that is almost exactly zero. Well, there's a chance that dlc (or any digital content) will work but the disc would have to run in order for the dlc to be worth anything. Due to the Ps3's C/GPU & OS architecture I don't see it happening.