Why must new telephone equipment be compatible with older equipment already in use?

In systems engineering, new telephone equipment must be compatible with older hardware because the telephone plant functions as a composite of various components designed at different times to meet diverse operating conditions.1

Because these disparate pieces must work together to maintain a functional network, any new apparatus must integrate seamlessly with the existing infrastructure. This requirement is central to the systems engineer's responsibility of establishing performance standards that ensure the orderly evolution of the plant, allowing it to meet both current operational needs and future service demands.1