There is no "last block". Miners will continue mining indefinitely (which is to say, as long as there is non-zero interest in Bitcoin). As the block reward (currently 25 BTC) shrinks to nearly-nothing and then goes away completely (circa 2140), it is transaction fees that will be the incentive for miners to continue.
There is a last block. The larges values of the variable that they use to store block ids.(this is very large)