No matter what they might call it, selling fresh copies of old software implies that they're obligated to support it. And I bet you'd see lawsuits insisting on it. No consumer expects to have a dead version with security updates if they bought it yesterday from the company what made it.
And seriously, ML is $20, L was $30, SL was $30. What exactly is a discount price? We're not talking $150 upgrades anymore. It's all hardware subsidies now.
Finally, because you seem to have forgotten, Apple is the closed sales loop. They have everything to gain from encouraging you upgrade hardware, and no reason to care if you choose not to.
Isn't there some kind of aftermarket for developer seeded OSX builds? Or did they solve that problem?