Using iPhone 5 support as a heuristic to identify abandonment is a great idea. It’s a black and white cutoff. Alas, in an environment where app stores are graded based on the number of apps they contain, Apple may be resistant to remove the stale apps completely, even though this is probably the action that is most beneficial to users.
At minimum, Apple should tweak the iTunes search algorithm to deprioritise the abandonware in the rankings.