It would be interesting to know how companies like Apple account for the market of older models and how that feeds into their services revenue. For example if you can go on Woot and purchase an iPhone 8 for $399, while Apple didn't make money off of that sale, they will certainly make money from app purchases and iCloud subscriptions. And at $399 there's bound to be people willing to purchase an iPhone even if it's older. With Apple's iOS support reaching back several generations, it's not always a bad deal.
So in a way, even though Apple isn't making money directly off the sale of that old iPhone, they're still able to feed this lower-end market and make money from apps and services, without directly lowering their prices and maintaining their perceived high-end status.
Edit: I gave an iPhone example but it certainly extends to the Mac lineup as well.
I made up the numbers as an example. It doesn't really matter, my point is that Apple's old devices, be it iPhone or Mac, still have value years later. So even when people are fishing the old stuff from the bargain bin sites, Apple is still making money off of them. As a result, Apple doesn't need to compete with the $400 Dell because people will still be buying the 5-year-old MacBook Air from the bargain site and with it, buying app/iCloud subscriptions.
So in a way, even though Apple isn't making money directly off the sale of that old iPhone, they're still able to feed this lower-end market and make money from apps and services, without directly lowering their prices and maintaining their perceived high-end status.
Edit: I gave an iPhone example but it certainly extends to the Mac lineup as well.