I have not worked on mainframes per se. But I was once tasked with investigating why a particular project to replace a mainframe process with a modern Java one

I have not worked on mainframes per se. But I was once tasked with investigating why a particular project to replace a mainframe process with a modern Java one failed. Basically, the mainframe is FAST. Everything is simple there. The process in question was more or less a COBOL stored procedure doing a bunch of lookups in an incredibly denormalized DB2 database. The entire batch job would take less than 15 minutes on the mainframe of which this particular process was only a small fraction. Now, the Java app to replace the mainframe process was apparently written by some smart dudes that had gone on to make lots of cash doing more exciting things by the time I got there. The replacement system was what lots of naive people drinking the Hibernate Kool-Aide were writing back then. The database was normalized to the extreme, it had a hierarchical object model that was mapped to objects with Hibernate, had a slick web interface, and so on. Well it ran like balls. It got exponentially slower as data was added. The administrative interface took 30 minutes to save a setting. This new process would increase the runtime of the overall batch by an estimated 10 hours. So I looked at it, and with another guy cleaned up a bunch of bad SQL and Hibernate. The administrative interface got shaved down to seconds for a post. But in the end the overhead alone of marshaling and unmarshaling XML for the service calls was enough to push the batch process a full two hours longer than the mainframe one. Which, by the way, was unacceptably long for the 1 hour maximum processing window allotted to the batch. OK, I know what you're saying. That's when you bring in the cloud and run a bunch of parallel instances and such and such. Or dump XML and use some kind of raw binary message format. But shit. The mainframe already did all that with a single threaded COBOL stored procedure in under 15 minutes. Needless to say, today they're still using that same old COBOL stored procedure and they're still making money hand over fist. So yeah. Big companies spend lots of money trying to get off the mainframe every year. Believe me, I have personally witnessed tens of millions of USD lost to failed mainframe replacement projects by developers who think they know better. It's not like some manager looked at the the estimate and said "hell no." Actually they said, "Hell yes! My top architects say the mainframe is dead. I will write a check for $50 million dollars right now because that is nothing compared to the $10 billion in revenue we make." It's just not as easy as it looks.