WPF via out of band NuGet packages is a great move! Seeing what it did for ASP.NET or especially EF, I can only see this being a good thing, especially if the investments aren't split between the "in-box" WPF and the NuGet package.
Although it does seem like WPF may never be truly cross-platform, it does seem like there is a good middle ground. Even if it remains Windows-only, what are the chances that it could become open source and sit on top of .NET Core? I ask because I have a solution that includes both ASP.NET MVC apps that I would like to update to ASP.NET 5 on .NET Core, plus WPF apps and Windows Services. I don't really have a need to run my app on *nix, but the other benefits of .NET Core are huge -- not having to ask each of our customers to install .NET framework updates would simplify so much.
I know I can use PCLs to share code between .NET Core-based projects and full framework-based projects, but frankly that should be a transient workaround. Now that WPF has taken the first step of moving to NuGet, getting it and its dependencies included in (or at least, compatible with) .NET Core should be a priority once the current crop of features have been addressed.