They can certainly buy new plcs and new hmis to interface together and do things but it's not like a program written for one plc from one manufacturer is interc

They can certainly buy new plcs and new hmis to interface together and do things but it's not like a program written for one plc from one manufacturer is interchangeable with one another. the plc code is fairly close to assembly language and it's written for one particular plc with a particular wiring. I could do it but it does take time and money to complete and even then you lose a bit of institutional knowledge codified by the original codebase. Having delved into plc programs built by others(even with comments that get stripped out if you don't have the original source file and instead download it from the plc directly) it's not easy to do. They certainly do still sell new plcs for industrial automation but the programs are typically bespoke.