It is 40 years to the week since the US government filed its last official antitrust complaint against IBM, and more than 50 years since it reached a landmark consent decree with the computer maker to open up the early computing industry.

Yet some technology monopolies never die. The market for mainframes – the heavy-duty, monolithic machines that dominate the high-end of the computing market – once attracted the attention of companies including General Electric to Honeywell.