Right, I agree with everything you said here. That's exactly the reason why MS said at the Build conference that they will be supporting .NET Framework "forever". There is a lot of it out there and it will likely remain for a very long time.
When I said there won't be a concern with portability via .NET Standard, I merely meant that the current shifting landscape of Framework features vs Core features vs the intersection known as Standard, will go away. There will be .NET 4.8 in maintenance mode and .NET 5 as the future, growth mode. Maintaining .NET Framework apps hopefully won't be a problem but if you're thinking of building a new app using .NET 4.8, you're likely making a big mistake unless it's to satisfy some immediate need that doesn't work on .NET Core. MS said they're committed to supporting people stuck in that position. Just don't expect new features that will appear in future .NET versions.