No, the issue is that so many developers assume (rightly, at the time) that the CSD won't change unless you call Wimp_Poll. And the same applies to dozens of other facets of RISC OS. I bet bags of applications rely on things like the CSD, sprite context, currently open Templates file, and lack of file system races. And they'll all break.
As the the file system, you don't need non-blocking APIs to make a performance improvement, you just need the ability to run a different program while another is waiting on disc.