Something trivial - like "hey, that function is failing... was it failing with last week's version as well?" - is very hard to arrange if you have any non-trivial dependencies. You have to build some homebrew lockfile mechanism (ugly!) and then you discover that most open-source mirrors don't keep old versions for that long, and so now you have to set up mirror as well... And then there is dependency resolution problems as you try to downgrade stuff...

And then at some point someone gets a great idea: "hey, instead of trying to get dpkg to do things it was not designed for, why don't we snapshot entire filesystem" - and then the docker is born again.