My time in large-scale systems taught me that it is seldom the financial price that drives that kind of decision making. At least not directly. Usually it is some kind of “what’s the effort versus payoff?” in terms beyond just Akamai invoices.
For the App Store specifically, that very mentality works against keeping apps available. From Apple’s (or for that matter Google’s) perspective, a production App Store has the front end, sure. Also the CDN, the entitlements, DRM, and code signing, payments and IAPs, interfaces to developer tooling/upload/analytics, scanning for the use of private APIs, and so on.
To make such a system function more like FTP for old apps, they’d have to build that in or at least build paths that serve to exempt classes of apps from some or all of that.
I can absolutely understand why their perspective is that if they’re spending effort to remove or deprecate old apps or features, it would appear crazy to introduce new features or services simply to serve up the old. Even if they could do it at cost, that’s developer time at least, and “why spend that on old stuff?” Apple is the company that razed its own corporate museum under Jobs because he thought the past was irrelevant or would hold them back.
I would LOVE if they did that though. They see the dollar signs involved in preservation, not the value. Sadly, the day may come when App Stores share the fate if the game stores like the Wii U storefront, and entire generations of software will be lost.