I work with people who work with lab software which is happily running OS X 10.2 Jaguar. It's no different than the XP box in the corner, as long as everyone understands that the checkbook has to come out when it breaks.
On the other hand, the quarter million dollar cytometer is going to be running Windows NT 4 Embedded, and when that ATA-66 hard drive fails or the software becomes unable to talk properly with the modern world anymore, it's on the vendor to make it work again. The support typically blows, but it's their problem to handle and none of them are going to be relying on Apple's memory-hole platform.
As long as Lion gets security updates (2 years, minimum), there's absolutely no need for an upgrade. After that, you can just disconnect it from the Internet and use an stronger password.
I completely agree and that's a very valid scenario. Apple tactical advantage is that consumers have completely different usage patterns and expectations than the enterprise/lab (which Apple essentially ignores). If %95 of consumers either (a) upgrade within five years or (b) are completely perplexed by the notion of updating their operating system at all, then Apple are at a huge efficiency advantage EOL-ing technologies around the five year mark. As opposed to having top engineers work on maintaining compatibility with deprecated APIs, old hardware, etc. for 15 year old software.
Hiring some maintenance programmers to take care of 10.6 or 10.7 security for X more years (and announcing that EOL date!) sounds pretty harmless. How would that hold Apple back at all? They have mountains of cash in the bank and those programmers wouldn't get into the way of 10.8 or 10.9 at all.
You can't just add programmers to mate cutting edge OS kernel features (ie leveraging recent videocards) with old hardware. None of us can speak authoritatively on the case in point, but it seems painfully optimistic to think you can pull this off without introducing a lot of complexity. And then, what happens if the 10.8 team tweaks the kernel during a point update? Yep, better double or triple the testing team.
"Oh, it looks like it'll take some work to get 10.8 working with all of our old AMD chipsets - either we delay release until fall (after Windows 8, unfortunately) or we put some of our "A" engineers on it."
Complexity is the keyword. Complexity and mediocrity go hand in hand. Backward compatibility is a noble goal in many ways, but every generation back of software/hardware you try to maintain exponentially increases the complexity you deal with instead of putting a better new product in user's hands.
HN completely ignores the class of computer users who only need Safari, Mail and some variation of Word, and then live in these apps for years without any major upgrade. In my experience, these people don't even feel entitled to any upgrade and they don't add any complexity to any testing matrix. All they need are baseline security updates, and existing services better don't stop without a plausible reason (see MobileMe). I'm not thinking of MegaCorp here, but rather my own and friends' close relatives.
The machine that got excluded from 10.8 after only 4 years was not a rev-A MacBook Air in the hands of some early adopters but a Mac Mini, the most traditional Mac money can buy. That really leaves little hope that all my "slow adopter" relatives will ever leave the Windowsverse.