[graycat]

Long one of the main reasons for IBM mainframes was the bet your business software that wouldn't run anywhere else and that would be too expensive to rewrite to run somewhere else.

Also, there is a remark that in major parts of the financial industry, running an IBM mainframe is nearly a necessary condition for compliance.

[jussij]

> would be too expensive to rewrite to run somewhere else.

I'm don't doubt that is a major factor. Add to that the major risk that what every new system you move to might actually fail to work or end up costing more.