[nickpsecurity]

"They'd have to fix all of the packages in the main repository, and then PPAs?"

The NixOS site made it look like they didn't have to do that with their solution: just give a meta-view on what's already there for the users.

"Lots of third party software targeted on Ubuntu would break; this would cause a lot of users to want to stay on pre-change versions of Ubuntu, which probably means it would get forked."

This could happen with any number of changes. Might still be worth it for at least major changes like making broken packages recoverable. I can't imagine myself getting off Ubuntu just because package failures can't brick my system anymore. Some developers might gripe about having to make changes but that makes them look like assholes, not Ubuntu.

I mean, VMS had this feature in the 80's with their versioned filesystem and transactions at app level. It's about time at least the package managers of Linux can do same reliably for apps. At least one of them did to their credit.