The app manifest could state "I need this runtime installed". The OS can know whether it is already installed or not. The OS also knows whether it is internet connected or not. If a runtime that isn't installed is needed and internet connectivity isn't available, the OS could state that, with information about how to procure the runtime manually.

That would stop people that have copied thr application from a pendrive in an air gapped system, that's true. One could make the case that someone meeting all of those requirements for would be technically minded enough to figure it out.

This wouldn't help developers who want to distribute a small application that won't ever require a download, but would be an improvement for anyone already distributing .Net applications over the web.