Managers and owners of HP 3000s are the kind of customers who understand what an operating system does. Most of us in the community remember when there were countless OS's out there to run our businesses, if not necessarily our lives.
The HP 3000 stands out in a healthy legacy comparison because its birthdate in the initial generation of minicomputers. Unlike nearly all, its OS remains in business use today. Other OS's which are not in use: MCP from Burroughs (a source of MPE inspiration); Univac's VS/9; NCR's VRX; Control Data's Kronos; and Honeywell's CP-6. 3000 veterans will recognize those as BUNCH companies, whose mini and mainframe products were swept away by IBM's, HP's, and Digital's.
MPE has not yet outlasted the VS minicomputer operating system from Wang Labs, since that mini still has support from its latest third party owner, TransVirtual Systems. There's more than blind loyalty there when an OS can move into the four-decade lifespan. There's commercial value, too. VS still has about a decade to go to get to MPE's 41 years.
For the 3000-savvy, the cartoon above would have a few extra boxes in it. The longest one is likely to be MPE, in its II-V, XL, and iX generations. There are a few others that pre-date DOS, of course. HP tried to sell PCs running CP/M, for example. You could insert the following boxes underneath the fine cartoon from XKCD, the work of brilliant cartoonist Randall Munroe.
That useful lifespan for MPE will run to 53 years, unless a rolled-over calendar is not a problem for your applications.
Hop over to Munroe's website to enjoy the irony and heart of someone who understands that Gnu (yup, the root of the 3000's iX generation) could be there at the very end, turning out the lights. And who can say for sure that MPE will truly end its days on Dec. 31, 2027 after all? Wang's OS has passed through several third party hands. HP's own VMS will become the property of a third party next year.
In-tribute plug: If you can't find something on the XKCD store to buy, or a cartoon to link to, then all of the above is probably nonsense. For the rest of you, let me know if Gnu could really rule the planet after civilization ends. We're already hearing that embedding a Linux microkernel would make the OS more useful for Digital server users. Something less complex is surely on its way. It might arrive before that fire.