I don't think we can completely replace clearnet for darknets unless it starts getting used by everyone.
Things like watching videos (let alone streaming) is still extremely anal, takes like 3 days to watch a 30 second video, or hosting a gaming server on the darknet will render gameplay useless.

Also, most of the Tor exit nodes happen to be in western Europe whereas most of the I2P routers happen to be in Siberia, so to me I2P is faster than Tor, but for someone in Europe Tor will definitely be faster, and people in America, and even moreso in AUNZ and Africa are just out of luck with both.