I think the idea is to throw away how the OS does things, not so much what it does. The more essential applications will use the operating system at quite a high level, and only need a relatively small subset of the features of RISC OS to be implemented on the newer OS.
The kind of thing that relies on the more esoteric features will usually be smaller and best re-implemented using the new APIs, anyway. (e.g. USB device drivers, filesystems, etc.)
Which RO applications really keep people using the OS?