In theory it's possible to let your dapp run forever, hogging resources from the network and "breaking" the system. But that would cost you such a large amount

In theory it's possible to let your dapp run forever, hogging resources from the network and "breaking" the system. But that would cost you such a large amount of money that no one does it. In other words, the system, while breakable in theory, isn't in practice because of a set of economic incentives (or as it were, dis-incentives in this case).