>most of the business software that goes into said mainframe is in fact very simple and straightforward

Yeah no. I worked on mainframe code for 15 years of the last 20 and can tell you that you’re wrong. Even reasonably simple business lines have cobbled together insane cobol, batch and cics screens that would make spaghetti blush.

When you open up a 100,000 line batch program that’s just 1 of 30 cobol programs in that run which is a mess of gotos, global mutable state and basically every single other generally frowned upon software practice we know about, you start to understand the issues.

Moving off of mainframe is practically impossible because of it. That other person being voted down is actually correct, nobody in the organization actually knows how any of it works.

When organizations try to transition away, what ends up happening is they get these insane promises of “5 years” from vendors (like IBM. Ask me how I know!), but then it takes 10 years to transition single systems.

Nothing is documented. So you have business people who have no clue how anything works providing high level work flows. But then there’s always 50 random edge cases or interactions they never considered cause the 40 year old code just does it.

IT contains most of the business knowledge, but the mainframe grey beards are… stubborn. The business hangs on to these people because they’re seen as insanely valuable, not realizing that the grey beards have actually been actively suppressing knowledge to only their hidden documents, and they’ve got 8 years till retirement and are going to make sure that their job exists at least until then by actively sabotaging efforts to change. But also, for some reason, the business manager love the saboteurs. It’s insane to watch.

I'm not sure how much water the multi tenant HA argument holds, these institutions are already doing that on AWS / Azure / k8s for the more modern apps. Rewriting all the old crusty COBOL code might be a different story.

The risk in a reengineering project that comprehensively replaces the mainframe is enough to discourage most people making that decision. It could be a $3M project that pays pack in 4 years after scrapping the mainframe and running out the maintenance contract, or it could go over budget and take forever to pay back. Your IBM rep has undoubtedly run the numbers.

>> It persists in many places due to the deeply ingrained belief that, for one reason or another, it would be technically, practically, or economically impossible to migrate functionality off the mainframe.

> It is never technical or pratical reasons, always economic reasons. To use contemporary vocabulary, you "just" need to replicate a "multi tenant HA cloud environment" to migrate from a mainframe at great cost.

When your business processes are so ossified by a mainframe that you have, say, groups preparing Excel spreadsheets -- with data formatted from one mainframe screen in 3 columns -- for use by another group to input back into the mainframe -- with the data reformatted in 4 columns -- you know you're never escaping the gravity well of the black hole at the center of IT that is "The Mainframe." This is just how it's going to be until the company is so technically irrelevant that the remains are picked up in a fire sale by some PE to sell off to some other, tangentially-related company, solely for the IP assets.