I find this a good idea. Rim's work is exceptional, deserves to be honored with money. I would gladly pay another $50 for the continuation of the development. The subscription model is not popular among hobbyists that don't earn money with music production. Hobbyists use their tools occasionally, not regularly.
Therefore, a good idea in my opinion would be an in-app-purchase that serves as a donation pot.
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Off topic, but related: Lemur is a modular OSC/MIDI controller that was ported to iOS. It costed $50, like Auria Pro. The company didn't make it through the years, price went down to $25, development stopped, the app didn't work anymore on newer devices. Old iPads from 2012 became precious because Lemur worked only on them. Then Lemur was bought by a new company and presented it at the App Store for $100 for purchase and $100 per month. Even the most enthusiastic Lemur users rejected this offer. Such amounts of money are only affordable in a professional way, when you make thousands of dollars per month using Lemur. The new owner had to give in and abandoned the subscription model. Now it is a one time purchase, $100, and you own the software. Again.
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I think that's the point. Hobbyists want to buy a license to actually own the software. Forever. They don't want to rent a software. They work in different fields and may abandon music production for years, just to return back and create a new piece of music with familiar old tools.
The appearance of Lemur on the iOS platform was the reason why I purchased an Apple product, the iPad. The appearance of Auria made it clear to me that the iPad is not just a toy. For me it is actually these two softwares that provide this tablet real practical value.
