The ABI is only part of backward compatibility. From another POV, you can run software designed for VT100 or other terminal in seventies on any Linux or Unix today, (though it may be recompiled. That legacy mentioned in previous post is that terminal is still considered serial device with specific protocol). There are companies, that do exactly that in production. You would be hard pressed to find DOS-era software that bangs on buffer starting at 0xB800:0000 that still works in 64-bit Windows.