For me, the mainframe is a big expensive box. Vertical scaling is still a thing. Most RDBMS work best on big boxes. Some Java applications would require hundred

For me, the mainframe is a big expensive box. Vertical scaling is still a thing. Most RDBMS work best on big boxes. Some Java applications would require hundreds of GB of ram. There are use-cases for mainframes but people prefer new shiny Kube clusters.