While it may be the case that pay-per-use would be a great model for developers (though I'm highly dubious that this is the case for anyone that isn't making a trivial $0.99 app), that isn't the main thrust of my hypothetical.
The problem with this scenario is the loss of a full quarter of revenue from every new App Store user in the interest of Apple acquiring subscribers. Do you think it would be a great deal for app developers to not be paid for three months of every single App Store user's usage?
But... to address your main point - yes absolutely, I'd happily give up a quarter of revenue to move to such a scheme, where my recurring revenue would be much higher because my apps are actually used by people and have a lower barrier for new users. It would be a mutually beneficial chance for developers, apple, and consumers.
However - I do grant that developers are more likely to be able to finance this transition than musicians.
Since nobody is paying for the music during the trial, and the intended result is higher revenues for the artists (and for Apple, but notably proportionally more for the artists), it is flatly disingenuous for the artists to position Apple as getting all the benefit.
What if Apple offered the artists an advance covering the 3 months based on current iTunes revenues, and then recouped this over three years by taking a higher percentage for that period from the artists who opted for it? That would solve the problem of the 3 month gap.
I use the Spotify app 24/7, yet I use Editorial and Pythonista (both costing around $7), an app I value to a much greater degree, sometimes just -8h daily. And the truth is, while OmniOutliner is so much usefuler than say Pinterest, people spend a lot more time on Pinterest.
Don't get me wrong, I do see why the losers are probably the 99¢ apps, after all, you probably do not use those on a concurrent basis. Yet, if we go by the hourly usage, people use social networks like Pinterest and Facebook, games like Temple Run, and music services, to a much greater degree overall.
I care about a world where the most useful apps are rewarded.