Actually this makes it easier. It expands the subset of the .NET Framework that .NET Core supports. It's the same APIs we have used forever, just more of which are now available on other platforms (.NET Core, Xamarin)

That's only true if Microsoft continue to develop .NET Framework as well as .NET Core.

If I don't want portable code, I don't get support anymore without learning .NET Core?

So far I haven't seen any indication that they plan to stop developing the full .NET Framework. I'm sure some parts of it will stop growing - things like WinForms and WebForms probably aren't going to see a lot of attention in the future, but they're also as close to done as software can be. Large enterprises will be using them for probably the next decade or more, even if they get no features at all.

I look at .NET Standard as a good thing. Even if you don't write portable code now, a whole lot more of what you know as the .NET Framework today will be available to Core as part of the standard should you ever want to write portable code in the future.

I think .NET Standard will eventually make it easier to describe what .NET is. Right now, the .NET Framework is way bigger than what most people think about when they hear the word framework. The .NET Framework is a pretty extensive library, plus Winforms, WPF, WCF, ASP.NET, ADO.NET, Workflow Foundation, and probably others I'm forgetting.

Having a robust stdlib in .NET Standard should simplify things. All .NET environments will have the same standard library, and the .NET Framework to .NET Core comparison will be something like the Java EE to Java comparison. I know that's by no means a perfect one to one comparison, but it'll help make it easier for people to understand the difference between the two main non-mobile .NET distributions.