alt Hacker NewsMicrosoft pulled it off with dotnet, sort of, because they approached it like a completely new language, sold people on the benefits of it.
And the people they were selling that to, ( It's free and open souce now! ), were a very different group to the market they left behind on .NET Framework, who are often still struggling to make the transition now.
Had they actually killed off .NET Framework, it would have been a different story, much more like the VB6/VBA to VB.NET transition, which so few people bothered with that VB.NET died out, because if you had to retrain that much, you figured you might as well go to C# or a instead, or indeed a completely different language entirely.
I briefly worked professionally on a VB.NET project, but outside that job I've never met anyone else who can say the same. I've met a few who went straight from VB6 to C# though.
The whole push behind .Net Core was to get .Net running on Linux well and natively and was being lead largely by those of us using .Net for web. We were being murdered by other languages at the time; mono was an option but Microsoft shops usually needed to target something blessed/backed by MS.
Largely the same market (Enterprise) but not the different segment (web as opposed to Windows/WinForms).
I ported about 15 years of projects from various versions of .Net to .Net Core whilst they were developing (and sent feedback to the team - they were asking us to do that) and the process was pretty reasonable. You were only really stuck if you were using something very very Windows specific (certain image processing libraries iirc) and even then it was largely manageable.
The old full-fat framework is, AFAIK, still supported, as there's a whole lot of legacy code which is Windows specific which is still expensive / hard to port over.