Well there are 38 Megatops and a maximum of 25 A380s (at the moment), coming in. So that leaves 13 Megatops at least. However SIA are currently looking into the 773ER as a replacement for the less efficient Megatop.
I don't think the A380 will kick all the Megatops out. The A380 is simply too big for some routes the Megatops are operating now. The Boeing 777-300ER may in future if it can carry as much as the B744.