but because the app is no longer in development, it's essentially useless

the app used to store data for up to 5 users to keep track over time. I miss that!

What? Was it storing the data on a cloud server? In that case it's a different story, but a local app should continue working essentially indefinitely.

All this focus on source code is IMHO missing the point. RMS also missed this point when he started the GNU project. Source code is neither necessary nor sufficient for (legal) freedom. They just need to relinquish the copyright and release any keys and such getting in the way. Lots of examples otherwise --- I'll refer you to the cracking scene, game modding, etc.

In the physical world, products can be "EOL" for decades and the aftermarket will fill in the void if there is demand, often even when the original product is still in production. The original manufacturer never released blueprints and other comparable-to-source-code information; they just don't try to stop the aftermarket. Mid-century cars are a great example of this.

tl;dr: stop demanding source code, start demanding freedom.