You do see how there’s a significant difference between my computer getting security support for another 2 years compared to another 4 years, yes? It’s twice as long a period of time.
Going from 8 years to 10 years doesn’t sound that big if you purchase your device on day 1. But for people who buy devices later, it can be the difference between only 1-2 years of support and 3-4 years.
I do. What it comes down to is that Apple's support cycle is based on giving many years of support for current (non-discontinued) machines (7.5-8.6 years, on average, depending on when in the product cycle you bought it).
As I mentioned earlier, when you buy a discontinued machine, you're making a choice to get fewer years of support, but at the same time should expect the product to be heavily discounted based, in part, on that.
That's why buying discontinued machines is a good deal, especially if they're still under AC+, and you're smart about shopping around (unless you buy them from the Apple refurb store, where the discount is negligible).
Your're saying you want to get a great deal by buying an older machine, while having years of support close to what Apple now offers for current models. I don't think that's going to happen, since Apple doesn't want to maintain software engineering effort on those older OS's for more time than they do now.
You're right that machines would stay in circulation longer if Apple offered longer support, which would be more green. But costs would also rise (if Apple wanted to maintain its same profitability). I'd expect that discontinued machines would cost more as well since, with more years of support, they would have more value.
You could get more years of support by buying a more recently discontinued machine but, as in much of life, if you want more, you need to pay more.