The whole point is that everything changes around software. Drivers, CPUs, GPUs, web browsers, OSs, common libraries, etc. Everything changes.

It doesn't matter if x86 is backwards compatible if everything else has changed.

No code can last 100 years in any environment with change. That's the point.

If you restrict yourself to programs that don't need an OS or hardware, you're going to be looking at a pretty small set of programs.

I don't, but I do restrict that you run it on the same OS as it was designed for.

The program may work fine on its original OS, and the OS may work fine on its original hardware, but for someone trying to actually run their business or what have you on the software these facts are often not particularly helpful.

Backwards-compatibility in OSes is the exception, not the rule. IBM does pretty well here. Microsoft does okay. Linux is either fine or a disaster depending on who you ask. MacOS, iOS, and Android laugh at the idea. And even the OSes most dedicated to compatibility devote a ton of effort to ensuring it on new hardware.