I know, it sucks... There are a couple of alternatives:
Use another plugin that achieves the same thing as T-splines, There is one called Clayoo. I'm not a big fan of Clayoo as it crashes a lot and the interface and usage are not intuitive at all. That being said, it gets the job done in a similar fashion as T-splines used to do. https://www.tdmsolutions.com/clayoo/
Another option is to use Rhino's native Subd geometry Type. Subd's are available in beta form in Rhino 6 (and even with more functionality in Rhino 7 WIP). More info here: https://docs.google.com/document/d/1BsNLtTj8s0J-7uBuuKDRWJq6VtOLe_WlgPi-oQ7SxoI/edit# Use the "_ToSubD" to convert a Surface or Mesh to a SubD geometry (this is a hidden command in Rhino 6). The problem is that there is not a lot of functionality (yet) in Rhino 6 to edit and manipulate Subd's (more commands are available in Rhino WIP 7). So I'd advise you if you want to use Rhino's native Subd's to download and install first Rhino 6, and then download and install the Rhino 7 WIP to get the most functionality. Rhino 6: https://www.rhino3d.com/download Rhino 7: https://www.rhino3d.com/download/rhino/wip