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Bottom line is that no one can predict the future. The death of the As400 has been predicted for many years. In reality the machine has died, but the name lives on. As the machine has matured, we just change the name: I5, iSeries, Power I, etc.
I believe the architecture is still the best available for DB applications. If we want it to be around longer we need to change the way we talk about it and we need to promote it. We no longer should be saying Libraries, Files, Records. We have Schemas, Tables, and Rows. (Now we have a “Modern” machine)