Many companies and their enterprise products go back decades with oldest being from 60's. Those are still updated and sold due to lockin. In general case, your point is probably true given natural advantage of open source plus how market kills off so many IT firms.

A lot of OSS software goes back nearly as far, emacs is 40 this year.

Didnt know it was that old. Still younger than MCP (1961) which Unisys still sells for tens to hundreds of millions a year. Proprietary has record so far if it's old and still supported. Quite a few of them in legacy mode.

Only because someone was willing to pay money rather than have to update their systems. Intel fab lines are the sole reason that VMS still exists.

However, is MCP actually running directly on modern hardware or is it being emulated through multiple layers?

Of course, you can use my own argument against me that the ports of Spice 3 and Magic to modern hardware are hardly the same program as 30 years ago.

"Only because someone was willing to pay money rather than have to update their systems."

The same reason for a lot of what's in open source. See PHK's A Generation Lost in the Bazaar for some truly unimpressive stuff.

"However, is MCP actually running directly on modern hardware or is it being emulated through multiple layers?"

x86 is emulated on a custom RISC machine. So does it matter when saying whether x86 is a proprietary ISA that still dominates desktops? MCP is proprietary software whose apps keep running for decades regardless of what hardware they're running on. Like other mainframes, the AS/400, whatever. I hope you're not looking for loopholes around the fact that proprietary software can last decades if it's mission critical, supported, and hard to move off of. It can and does in quite a few cases.

"Of course, you can use my own argument against me that the ports of Spice 3 and Magic to modern hardware are hardly the same program as 30 years ago."

I can't since I don't know enough about them. The mainframe and futureproof markets for software try to maintain backward compatibility with stuff you don't change. So, the part the app runs on is probably the same with who knows what underneath. Sometimes that part is ported, emulated, virtualized, etc. Varies. API and software above stays the same.

Watch out for survivor bias. For every old but still supported platform I'm sure there are dozens that aren't.

Remember the original comment you responded to agrees that OSS wins here mostly but proprietary software can last a long time. And the oldest so far look proprietary. Survivor bias doesn't change that claim.