You are kidding right? There are loads of old systems that have not been updated, running on MS-Dos or Windows 3.1 systems, stuck in some cupboard in a factory, necessary for continued operation. Stuff that 100 people have ever heard of before.
Yeah, I know about those. There are industrial control computers that still run on Intel 386 (the original). But once those guys run out of spare parts they won't just upgrade the processor.
As the URL suggests, there are applications where an existing, well-characterised CPU is highly preferred over a relatively new one where some previously unknown errata could have catastrophic results.
It doesn't even have to be errata. That timing loop someone wrote decades ago might run too fast now. Even if it determines its loop counter at startup, it may fail because it assumes 16 bits is enough for the counter.
Conversely, some instructions may have gotten slower over time. For example, I doubt intel worries much about the performance of the original 8087 floating point operations anymore, so they could move them into microcode or even make them illegal instructions, to be emulated in software (I don't they do at the moment because the overhead of decoding the original 8086 instructions is low, but if x86 stays around for a few decades, at some time, I think they will consider doing the latter)
8080, or 8086. They use older processors because the wider process has a lower chance of a bit soft changing. Which you really care about in say PLC's.
