The cards expire ~one year after the first purchase. At that point you'll have up to two years to transfer the balance to a new card. I don't see the conservation.
Octopus cards never expire, as far as I can tell.
The US is, not surprisingly, way behind here.
Because of the unreliability of MetroCards and MetroCard readers, the MTA is planning to phase them out in a few years and replace their card readers with equipment that can directly read contactless (RFID) credit cards.