Comment by pjmlp

Many of those machines are running Java workloads, also COBOL isn't the only mainframe language, and the best thing in terms of security is that they don't use C as systems language, rather saner stuff with proper arrays, strings and bounds checking.

That is why Unisys ClearPath MCP is still a thing, tracing back to its Burroughs 1961 heritage, security above all.