Comment by degamad

A few different options, depending on the client. One went to a competing "commercial off-the-shelf" product. Another hired a company to build a system in a low-code application platform running on .NET.

Yet another went to a different mainframe system that they already had. They had multiple systems (both mainframe and Unix) which did similar jobs due to acquisitions over the years, and they moved the still-active customers from the most expensive mainframe to one of the cheaper mainframes.