Hardcore characters do not have their world deleted and there is nothing stopping someone from re-entering a world with a new hardcore character. If you didn't discard all gear as you outgrew it, your new character would even get a solid headstart on the game. The only difference is that the gear you were carrying will be deleted (unless you're on a server) and your HP and mana are reset to zero.

You seem to be under the impression that mediumcore characters lose all their items when they die and then go safely farm for replicas. I'm under the impression that most people who play mediumcore gear up in whatever spare gear they have lying around and then go on a quest to save their gear, which can lead to some interesting difficulties (eg you've recently started hardmode and died and now your best available gear is mid-normal mode so the quest to your gear pile is more dangerous than it would have been before you died, but you really want those rare/nicely modified items back).