If you want to start a project from scratch, sure Core/5 is nice.
As someone who has been working with .NET since the first release: tell me about it! I fully concur. I don't deny this, nor excuse it.
That said, I believe that once Microsoft decided that it was OK to break compatibility, they managed to improve upon lots of things which up until that point had become fairly painful on the traditional .NET Framework.
You win some, you lose some, I guess?
Anyway, I do hope they succeed and the .NET 5 unification goes smoothly. I want those C# 9 features - but I also need some old third-party libraries to keep working.
But things seem impossible for some new features, for example async steam just need new feature from core CLR.
So after .C#8, they decide only to support it on .NET core
The goal for the initial .NET Core project was to have something cross-platform, modular and lightweight to build Linux-deployable cloud-services with.
Once that reached a mature state, they could expand scope.
And now, after .NET Core 3.x has landed, it has matured so much, that MS can see one single unified path forward which also includes some things which previously was only possible using the traditional .NET Framework (4.8).
So now they are adding those use-cases to the "Core" package as well, without compromising on what .NET Core was supposed to deliver. I honestly don't see anything wrong with that.