Given that VRPC on a modern PC at least has one concrete advantage over new native hardware (speed), I'd say it will potentially have had to have led to lost sales for native hardware manufacturers. Although I'd concede that probably the vast majority of sales come from users who'd either stick with their RPC until future hardware became cheaper, or who would have entirely abandoned the platform.
Also, the existence of a faster way to run RISC OS will also be a driver to the native hardware manufacturers to provide faster hardware. Until VRPC came out, many were happy to stick with their trusty old RPCs, so there was *no* market for faster, newer ways to run the OS.
That said, to deny that VRPC could compete with native hardware in any circumstance is shortsighted.