I think the real issue is when infrastructure things move.
How would the move to PulseAudio have worked in a rolling release? Would someone with a new install end up with PulseAudio, but someone with an existing install, would just never have it installed unless they knew what it was and started poking around for it?
The same for the transition to using evdev for (e.g.) mouse-handling in X.org rather than defining things in xorg.conf.
I know that Debian has lived through these transitions, but my impression was that the packages were setup in a way that allowed people to keep the old config, or move to the new one. That said, it seems like these issues could be more complex than a 'human being' (Ubuntu's target market) would be able to decipher.
It's not even clear that the rolling release will be intended for non-enthusiasts at this point, however as a matter of policy you can be pretty sure that any update applied to the rolling release will need to be tested against users as old as the last LTS.
That means the same sort of transition of config files and libraries that occur on release upgrade would just happen during a normal package install, which is pretty much how it works anyway (a release upgrade isn't much more than a giant group of packages being upgraded together -- all this logic is stored in the packages themselves).
I get the difference between a release upgrade and a rolling release schedule. My point was that when you are creating a release, you can say "this is the point where this change happens." With a rolling release, will a "apt-get upgrade" transition me? What if this was unexpected? What if this breaks my config?
For most normal upgrades this doesn't matter because the applications are usually fairly insular, but when you get into integrated systems like Desktop Environments it's different. Especially when Ubuntu is making a number of UI/etc changes to the desktop. It would be a bit surprising to have UI parts shift around. When you do a release upgrade, you know that there are going to be some major changes, and there are ChangeLog/notes on the release itself to tell you what major changes there are.
How would the move to PulseAudio have worked in a rolling release? Would someone with a new install end up with PulseAudio, but someone with an existing install, would just never have it installed unless they knew what it was and started poking around for it?
The same for the transition to using evdev for (e.g.) mouse-handling in X.org rather than defining things in xorg.conf.
I know that Debian has lived through these transitions, but my impression was that the packages were setup in a way that allowed people to keep the old config, or move to the new one. That said, it seems like these issues could be more complex than a 'human being' (Ubuntu's target market) would be able to decipher.