There are ways to ensure that required versions of the framework are installed when using an installer app. If they're not installed, they get downloaded and installed as part of the install process, so it shouldn't be too much of a concern, as long as there is some kind of check during installation.
The larger issue with supporting very old AutoCAD releases, is that some of them were released before the AutoCAD .NET API lapsed into 'maintenance mode', which means there are significant, breaking API changes that must be accounted for. There have been quite a few breaking changes over the years, that have caused a lot of headaches for myself and other developers. Examples are releases of AutoCAD that predate the separation of the API (e.g., AcCoreMgd.dll, which was often referred to as the big split), and the transition away from the fiber threading model.
Autodesk could have resolved some of those issues internally and avoided breaking changes, but unfortunately they chose a different path.
So I'm not sure what the fuss over framework versions is about, considering all of the other headaches that come with supporting very old product releases in a single unified codebase.