Eh? I literally gave an example - asking the user to pick a file or open a hardware resource (audio stream, for example, or USB device). The trusted system process shows the file picker & opens the file, and then just gives the requesting app that open FD.

It's a pretty straightforward & extendable way to do permission handling, rather than needing the kernel to do all of that natively. Whereas if you go the RPC route, then you're into not just a monolithic kernel but a larger-than-ever monolithic kernel as it's now doing permission management, too.