That's because users paid for it.
Google gave the world high-quality, free open source code hosting for eight years. I don't see how that's anything but a net positive.
I'll give you three guesses as to how much those companies still running OS/2 were paying for support in 2006.
When IBM buys a company/product, they will merrily throw everything out and declare that v.next is the next version of whatever product you are using. Then the legacy product goes in life support.
OS/2 lived on because it was widely deployed in banking for ATMs and POS, and it generated a lot of complimentary revenue. So when a company like Target had 100,000 cash registers, IBM got to sell services around deployment and maintenance, and management software like Tivoli to manage the devices.
'Eternity' overstates it. Plenty of vendors support projects long after they are obsolete. IBM supported OS/2, released 1992 and a marketplace flop, until the end of 2006.
That means that if I need something to be around for awhile, I'll count on IBM and not Google. Google has the perogative to make that choice, of course, and long-term support generally is associated more with the business market than with consumers. Party that's because it costs more for businesses to change, partly IMO because consumers are poorly educated customers.
EDIT: It's a little odd to see this very normal comment modded down to -1 (now -3!). Another perfectly fine comment was modded down to 0. I don't care about the points at all, but it supports my instinct that some on HN are trying to protect Google. Employees? Fans?