Earlier quoted context omitted.
If that happens, all someone needs to do is repackage Debian's version of aptitude for Ubuntu. Or an older version from Ubuntu. Really, this is a non-issue on Linux, because either 1) someone will just "fix" it an release their alternative, or 2) everyone will just stop using the offender.
Depends on what type of software you are talking about. For copyright protected proprietary software (which Canonical want people to develop for their OS) there may be legal protections which prevent it from being rebundled outside of torrent sites (which have their own risks). #2 assumes a savvy enough userbase.
If you're packaging something as a deb, you can potentially bundle other things, but at the same time, because a deb is a glorified tar.gz, someone can just provide information on how to get rid of the offending thing.
Or, if you're using Arch, the AUR maintainer just adjusts the PKGBUILD to do it for you.
It's not particularly hard.
And sure, having people install a different package manager requires a savvy enough userbase, to an extent. If they can copy and paste a couple commands into the terminal, they can change it (wget [somefile] && dpkg -i [somefile]). How hard.