What competitive technology are you thinking here? IBM z-series mainframe carry enormous amounts of transaction workloads today. They are deeply embedded in most of the Fortune 1000 companies. Hitachi was the only real competitor left standing and they dumped their mainframe business in 2017 since they were unable to get traction outside of Japan.
In the early 90's, there were efforts by customers to get rid of their mainframes which put IBM on the ropes then, until Lou Gerstner took over as CEO and stabilized the business. It's not like these transaction workloads can be easily ported elsewhere since some of them go back as far as the early 1970's. Those customers able to ditch their mainframes did so a while back. The rest of these systems will remain due to risks and costs of migrating to a suitable alternative. It's not a huge revenue stream for IBM, but a very profitable one.