On the upside, this is still being worked on. The UBports community is still active, and KDE has been refining its plasma mobile interface. Purism's librem 5 should be convergence-capable.
Window toolkits are also being developed to help create responsive applications: Kirigami and libhandy, to name a few.
On the non-opensource front, there is also Microsoft's UWP, and samsung's DeX is showing some promise.
We're getting there, but slower than anticipated. The EOMA68 is also a promising concept, though slightly different, and more similar to what the parent describes.