The problem with the strategy of shifting to a more annoying system is that it may lead to a vocal minority starting enough of a review revolt to push users away who would have otherwise not been bothered by the ads. A modern developer looking to make some money off mobile apps needs to do more then just be a programmer. They also need to know how to market, or at least be able to have enough good reviews to bring in more users. More users means more income, be it for a paid app, or a free with ads app. If the app starts off successful, then does something to annoy users, it's worse then doing the annoying part at the beginning before the app had any traction.

Google has been very successful with ads due to their great balance between providing enough enticing features for free, while serving revenue generating ads that aren't annoying enough to detract from the free benefits. For example, I can't see Google moving to a model where they force users to watch or look at an ad before logging into GMail or when running searches. They do however go for that strategy in video, as users are already more accustom to that kind of ad with video content.

Angry Birds managed to also do well by generating a ton of hype from their paid iOS version, then going for an ad supported version on Android. Had they not been successful on iOS, I doubt they would have been as successful on Android based on how annoying the ads are.