We understand that the discontinuation of Telerik Platform has a big impact on your development process and this is why we are offering a way to migrate to a different framework (Cordova CLI, PhoneGap) in the hopes of making the transition as easy as possible.
Unfortunately, we are not planning to open-source parts of Telerik Platform as their back-end infrastructure and code is still being used in our NativeScript tooling. And without the cloud infrastructure, the AppBuilder Windows client is unusable and even if it is open-sourced it would not be of any help.
I hope this information is of any help. Please, let me know if I can be of any further assistance.