IN 1991, Stewart Alsop, the editor of InfoWorld and a thoughtful observer of industry trends, predicted that the last mainframe computer would be unplugged by 1996. Last month, I.B.M. introduced the latest version of its mainframe, the aged yet remarkably resilient warhorse of computing.

Today, mainframe sales are a tiny fraction of the personal computer market. But with the mainframe facing extinction, I.B.M. retooled the technology, cut prices and revamped its strategy. A result is that mainframe technology  hardware, software and services  remains a large and lucrative business for I.B.M., and mainframes are still the back-office engines behind the world’s financial markets and much of global commerce.

The mainframe stands as a telling case in the larger story of survivor technologies and markets. The demise of the old technology is confidently predicted, and indeed it may lose ground to the insurgent, as mainframes did to the personal computer. But the old technology or business often finds a sustainable, profitable life. Television, for example, was supposed to kill radio, and movies, for that matter. Cars, trucks and planes spelled the death of railways. A current death-knell forecast is that the Web will kill print media.

What are the common traits of survivor technologies? First, it seems, there is a core technology requirement: there must be some enduring advantage in the old technology that is not entirely supplanted by the new. But beyond that, it is the business decisions that matter most: investing to retool the traditional technology, adopting a new business model and nurturing a support network of loyal customers, industry partners and skilled workers.