Most protocols will be phased out for security reasons. So I would go with direct TCP/UDP based on Linux kernel syscall (still the weakest point) in C or with fetch in JS.
Most operating systems will be trivially vulnerable to attacks, so I would go with the minimal possible attack surface (hardened kernel, firewalled, etc.)
It should survive and TCP/UDP communication makes it easy to integrate with any other software.
For the hardware I would go with a Raspberry pi if possible (and in C), otherwise just hosted on AWS.
About its usefulness, that would be hardest point. If it was useful and not updated for 5-10 years, it will be replaced however complex it is, 100 years later the problem might be generalized or not relevant anymore.
Looking at what's there:
- 61 years old, MOCAS, OS written in COBOL used US Department of Defense, on a mainframe
- Linux, Python, glibc, perl, emacs, ...: Most old software still relevant is a core foundation (programming languages, operating systems, text editors)
- NASA's software: C, C++, Ada, Python. Linux for applications, Windows for end users.
So I would probably make a minimal programming language, maybe some kind of minimal Crystal.