Do you think 21700 battery will replace 18650 battery?
Yes, eventually. I don't think people are going to run out to replace their current hardware, chargers, etc., but when it comes time to replace them I can see people choosing the 21700 over the 18650. A lot of mods, including the Pico 21700 and the Releaux RX2, support both, so that will help in the transition for sure.