> In reality, they probably could have updated the mainframe systems to be Y2K-compliant.
Legacy systems weren't always mainframe systems and, in any case a “legacy system” is precisely one you are no longer confident you can safely update for changing business requirements.
A change in a pervasive assumption touching many parts of a system (like “all dates are always in the 20th century”) is precisely the kind of thing that is high risk with a legacy systems.
Story time: years ago (2010-ish?), we were doing some light web CMS work for a client. Nothing too complex, except we had to regenerate one section off a data feed that we received in a very peculiar custom format.
Great, everything worked, we finished on time and on budget. And then: "oh, and could you also check this other site? We need some minor tweaks on what your predecessors made". That other site was also consuming that data feed, so we took a peek. It ran in PHP3, walled off from everything, processed its own intermediate languageS, and the output was a massive 200+ page PDF (which was then manually shipped to offset print in large quantities). For Reasons, this had to run daily and had to work on first try. There was no documentation, no comments, and no development environment: apparently this was made directly on prod and carefully left untouched.
Needless to say, the code was massively interdependent, so that the minor tweaks were actually somewhat harder. We did manage to set up a development VM though, and document it a bit - but last I checked, the Monstrous Book Of Monsters Generator still seems to be chugging along in its containment.
And don't get me started on Oracle refusing to provide updated y2k compliant backports!
Sounds like some of the benefits from the Coronavirus crisis, like the increase in working from home.
For example, at a former employer of mine, a big justification for getting rid of the IBM-compatible mainframe and a lot of legacy systems which ran on it (various in-house apps written in COBOL and FORTRAN) was Y2K.
In reality, they probably could have updated the mainframe systems to be Y2K-compliant. But, they didn't want to do that. They wanted to dump it all and replace it with an off-the-shelf solution running on Unix and/or Windows. And, for reasons which have absolutely nothing to do with Y2K itself (the expense and limitations of the mainframe platform), it probably was the right call. But, Y2K helped moved it from "wouldn't-it-be-nice-if-we" column into the "must-be-done" column.