Data in older programs was in legacy formats that often could only be read by that program (and sometimes only specific versions of it). One might assume that

Data in older programs was in legacy formats that often could only be read by that program (and sometimes only specific versions of it). One might assume that since we're using a lot more plaintext formats, our formats and encodings today will last forever, but I wouldn't be too sure of that. Better than it used to be, but it would be hubris to assume that we've hit the pinnacle of perfection of the best possible formats. And yes, I really want to be using 20 year old software. I have software dating back to some originally written in the 1960s, updated in the 1970s, ported to the PC during the 1980s-early 1990s, which still runs fine today. Some of the programs require an emulator or virtual machine, but they still run fine. With a fair amount of confidence, I can modify and recompile the source code and it will still run. (I did this recently to add JSON output to a program written in 1989-1990.) The problem is dependencies. Originally they were compiled in or included. But then people got all wonky about code re-use and shared libraries led us into DLL hell. Then we had things like OCX and VBX components that were shared and then vanished. Much of that is patched-over now to avoid some of those problems, and for things from that era, a VM typically runs them. But more recently dependencies are much greater, and changing much faster, and much more often rely on network connections to specific endpoints which may not be there tomorrow. You can fairly readily pack some DLLs into a VM and run stuff from there consistently, but you can't pack a working snapshot of the entire internet into it. That's aside from how much more complex the build systems and their dependencies have become. I know for a fact that I could still easily build and run code I wrote >15 years ago today, but not code that I wrote 10 years ago. Not all of the things necessary to build it are still available. Admittedly systems like unemployment that are in daily use for business purposes probably should be updated. But if I want to play the original Fallout, released in 1997, I should still be able to. And in fact I can. Newer games might not last 5 years. If I want to go back and look at/adjust some vector images I made in the 1990s-2000s, I can do that in a VM, using the source files in the software used at the time, and generate new output files. With today's software, I won't be able to in the future.