Yeah, the breaking changes that accumulate over the years is the main nagging thing I have to keep up with. Since I use my apps myself, and tend to use beta iOS releases, I can usually catch these pretty early, but not always.
As an aside, I find it a lot easier to bring in consulting gigs if I aggressively open source tools that I make, and then promote those OSS projects. I find it more pleasant to tell people about free tools than to just promote myself alone, but they end up feeding into each other. Just my two cents.
First, wow, I didn't expect any HN readers to actually know about, much less have bought, my work! Thanks.
Second, I understand your frustration. I love building things, and sometimes I wish I could do that full time, but grad school is a 70 hr/wk commitment for me.
If the app is not useful for you without the missing feature, Apple does allow returns. I'm not quite sure how it works, but I see a few returns a year on my reports.
Also, I've open-sourced the hardest part of the app, the audio managing aspect, as Novocaine (GitHub.com/alexbw/novocaine), and along with the great package NVDSP, anybody could replicate the basic functionality of my apps with a few weeks of learning and effort. It'd be great for the audio app ecosystem, too!
I'm also interested in hiring a part-time developer to help flesh out the top-requested features, if anybody has ObjC coding experience. That'd make many more updates possible.
I built all of these for fun in college, and I've occasionally updated them afterwards. The only thing I do now is answer a few emails a week.
I've since gone back to grad school, but the yearly income has not changed, and approaches my stipend (low 5-digit).
What's been really neat is how people have found unexpected ways to use the apps. Sound engineers for halls and communities use Octave to set up the sound for concerts. Teachers use oScope to help kids understand how sound is composed of moving pressure waves of air, and how pitch is the frequency of these waves. Also, oScope had a tiny cameo in the show Homeland, as a "fancy science-looking analyzer tool for spying on people" (uncredited, unfortunately).