In the real world there are massive amounts of code in the .net framework. Because of the significant breaking changes between 4.x and core/5 it will take a long time before all this code is ported, and many application will probably never be ported, while still having to be maintained. So for the foreseeable future it is going to be very important whether libraries are 4.x compatible or not.
Consider Python had (IMHO) fewer breaking changes between 2 and 3, but still the migration took about a decade.