People relying on features that aren't in .NET Core can keep using full .NET, and that's the intended result. Full .NET is going to be supported for the foreseeable future.
However for those projects that don't need them .NET Core is an interesting alternative especially with cloud deployment to Linux or a BSD host.
I work at a large .NET based company whose core product could not work on Core, but we certainly are considering it for newer projects. I'm also many years past being a junior developer.
Well, it's fine if features are ported later or in seperate packages or in some other/better format, but what I understood around System.Data is that vital pieces are missing, without a clear vision on how to deal with these. And if they are not added before 1.0 it may have far-reaching consequences. See my comment below for references.
The data story in general for Core is something they have repeatedly said is going to be worked on incrementally. Entity Framework Core is explicitly being sold as a "use only if you have to run on Linux/Mac" otherwise stick with EF6.
They're also making it very clear that features can and will be added/ported to .Net Core in the future based on what developers need and want. The thing they are absolutely keen to avoid is any breaking changes from 1.0 onwards - the plan is that you'll see 1.1, 1.2, 1.3 not a semver 2.0 for a very long time. To that end the legacy of DataTables is "how can we do this better" not trying to support decades of old libraries from the odd. The 4.6 Framework exists to do that and isn't going away if that is what you need.
However for those projects that don't need them .NET Core is an interesting alternative especially with cloud deployment to Linux or a BSD host.
I work at a large .NET based company whose core product could not work on Core, but we certainly are considering it for newer projects. I'm also many years past being a junior developer.