Interestingly, it occurred to me that NixOS might be in perfect position to enable painless "gradual migrations" of tooling.

Imagine alternative history of UNIX, where NixOS was invented really long time ago, before 'less' was created. Now, imagine we're Mark Nudelman and we want to add fancy features to 'more' (instead of creating a new tool named 'less'). We could do this freely, and all tools depending on peculiarities of "old more" could be pinned to use a "private" old version of the binary, while new ones would by default use the "new more". Then, the backwards tools could be gradually updated, until no one imported the "old more" and it could be safely removed from the nixpkgs repository. (While any external, non-nixpkgs package definitions could still backport the "old more" definition should they really need it.)

Does this reasoning have any holes?