The success in the IoT will depend not only in a cheaper price because of no royalties, but also in the "ecosystem": peripherals, buses, etc. Running Linux is a huge start, so I have no doubt it can be a success in this field.
Regarding the use in mobile and desktop, it will have to wait until SIMD extensions are introduced, and software being adapted (e.g. ffmpeg/libav including RISC-V assembly SIMD implementation for the codecs).
Anyway, realistically, for the RISC-V getting enough traction, some big player should bet on that, which is currently highly improbable, unless some Apple/Samsung/Huawei/Google gets crazy enough for doing it.