There are two scenarios:
1. Monthly subscriptions earn more revenue for developers compared to pricing sporadic upgrades/updates.
2. The converse of 1.
If 1 is true, then we’d likely end up with subscriptions regardless or apples incentives here. If 2 is true, then Apple is clearly leaving money on the table, because they take a percentage of revenues, and as parent comment states, they take a higher percentage of that revenue in the alternative scenario.
So it doesn’t really matter if they know which drives more revenue. All that matters is that the incentives dictate that Apple benefits the most when developers make more revenues.