I do not believe there will be a successor to the CD, unless it's another physical medium, perhaps one which is like a 120 channel multi-channel. card. After all, the CD can be re-sold, traded, copied, and be archived with more certain recovery than digital files. Also, although, SACD might be the defacto hi-res medium today for stereo or multi-channel, it has not been perceived as sounding better than CD by most making comparisons; and, it is still hard to get.
Certainly, downloading from the iTunes Store and others like HDtracks is convenient and delivers a product which I think is indistinguishable from the CD sound; but, what do you do with the digital library when you're tired of it? Is it likely anyone would buy it? And, is it legal to sell it? Does it just go in the trash? As I understand it, folks purchasing downloads are just purchasing a right to listen to the material. I don't think I can will my iTunes library.
One other thing, organizing a digital library is not the easiest thing. There are so many file formats, and although by one means or another you'll be able to play all of your files you may need more than one player to do it. There's also the upsampling/downsampling thing. When recording to DAT as I sometimes do, which only accepts up to 16/48. I need to go into sounds in Windows settings and reset for 16/48 or the DAT Recorder simply does not record.
The bottom-line for me is I think it's best to buy the CD and rip to a players music library. Then you've got it all, for yourself and others.