Sorry, but that would be stupid. From everyone's point of view. Let's say, hypothetically, that lots of people signed up for this. Now, you're a developer and you make a new app. How are the subscription dues divided? Is it per download, or are there greater shares for more expensive apps?
1) Per download: people who make "stupid" little $.99 apps like Koi Pond would get tons of cash because they would see a high number of "I'll check it out just to play with it for 30 seconds" downloads. Apps that are more expensive, say a $199 medical database app where there are a very limited number of potential buyers would get very little money even though their app was costly to create.
2) Shares by cost: if there were a significant number of subscribers I would release lots of gimmicky little programs like Koi Pond but price them at $999. No one would buy them, but the people who subscribe would download them and I would get a large value download.
What makes more sense is for Apple to build a time bomb trialware system into the SDK. Simply let the dev decide to offer a full featured trial version and pick a timeframe between some set values, say 1 hour and 7 days. That would pretty much do the same thing you're talking about but free on the user end and lead to increased sales on the dev side (I know that's one of the reasons I haven't paid for ANYTHING in the App Store yet - lack of good demp versions).