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.
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.