Ethereum promises that contracts will 'live forever' in the default case. And, in fact, unless the contract contains a suicide clause, they are not destroyable.
This is a double-edged sword. On the one hand, the default suicide mode for a contract is to return all funds embedded in the contract to the owner; it's clearly unworkable to have a "zero trust" system in which the owner of a contract can at will claim all money.
So, it's good to let people reason about the contract longevity. On the other hand, I have been reviewing some Ethereum contracts recently, and the code quality is somewhere between "optimistic as to required quality" and "terrible" for code that is supposed to run forever.