You said it, legacy. Our ERP is essentially a folder of three hundred or so programs that all `RUN` each other and operate on flat files. The coding is mostly maintenance, but we sometimes need to add new programs/modules/menus due to changes in the business. We will be replacing it at some point, hard to say when. I won't miss trying to understand a 30-year old program with line numbers and two-letter variables, but I will bet money that whatever the new system is will be slower and more cumbersome for users.
Does 'RUN'ning a program keep global variables and the like around (like CHAIN)? If not, then those pieces of code are self-contained and could be replaced gradually by something saner.
Outside of the SV bubble, it's not common for companies to migrate working systems to something new unless the benefits clearly outweigh the costs, and there's nothing better on which those those costs could be spent.
There are a number of stories online, in HN, and in the business press where companies "upgraded" from legacy systems to something "saner" and it ended up costing more time, money, and customers than it was worth.
