You're right, the tang is the metal that runs down the middle of the handle. If the tang breaks the blade slides out of the handle and you're left holding a piece of wood with a tsukuba at the top.
Even well made katanas are less durable than a western sword, that's true. But they can take more of a beating than some sword shaped piece of base metal.
And unless the sword was actually made in Japan, katanas tend not to be very durable in actual combat.
Even well made katanas are less durable than a western sword, that's true. But they can take more of a beating than some sword shaped piece of base metal.