Back in the day, POS/ERP systems were custom-made. So they are likely to be extremely well-tailored to the way a given corporate actually does business.
Replicating that in a more modern setting nowadays might either involve buying into SAP's racket and still paying huge sums of money for customization to get what you need, or building a new completely custom-made piece of software. -- There is an attitude in business that I find very lamentable but that is nevertheless a reality in that, nowadays, a business would never in a million years choose the latter course of action (i.e. a non-tech business like a chain of furniture stores would, nowadays, be extremely unlikely to pay for developers' time to build an ERP for furniture retail for their exclusive use).
But back then, that was the only thing you could do, and people did do it, because the gains in efficiency that could be reaped were just so big in relative terms (if your old system was pen-and-paper, it's a huge leap forward). Those low-hanging fruit are gone now.
The thing is: Those investments that were made back in the late 80s / early 90s continue to pay dividends to this day in the form of money that you don't have to pay to SAP. And this is a considerable amount of money if it means having to go out and hire consultants with staggering hourly rates each time SAP ceases support for the version of their product you're on, with data having to be migrated, customizations having to be re-invented, etc etc (The consultants, BTW, have to turn around and give a good deal of money back to SAP for certifications etc).
I think it's exactly what I would do if I was CIO in a corporate like that with a custom-made ERP system from the 80s. My most-preferred option would be to pay for some software developers' time to build a new one. But the likely outcome would be that I couldn't get a mandate to do that. So my second choice would be sticking with the system we have, and try to make it last for as long as it can possibly last. Only when that is no longer an option would I ever consider buying a modern ERP off the rack.