Of course growth numbers may suck, given how hard it is to make it for any app these days. Long-term growth would probably actually not be sustainable because if it blows up, there is no additional revenue down the road from lack of subscription, and also no network effects to power exponential growth in terms of market share. As far as I can tell, if you're not going to do subscription, there's no way to force users to pay to upgrade to a new version (new features) in the Apple App Store, not sure about Google Play. Your only option is to maybe create a new separate app that's able to import data from the old app, but that seems tacky to me. So, this really wouldn't be a big long-term play at all. But I imagine that there must be short-term markets out there willing to pay for apps that don't keep their data and usage hostage.
I'm myself thinking of making an app like this for budget tracking, just because I haven't found any out there these days that don't require neverending subscriptions and also fit my unique needs. Cloudless is also fine for me, as my phone is storage enough, or if it isn't enough, maybe use my iCloud storage, Dropbox storage, Google Drive, OneDrive, etc for holding the data? Besides that, it'll finally give me the kick in the behind I need to finally learn Swift, which I've been meaning to do for a while. Alas, I imagine cost of customer acquisition may be too high to make even a short-term profit.
So... would this just be a small project for me to prove I can learn Swift and show future employers that I can make smartphone apps too? Or is there actually a real business here and in so many other niches because some people hate being locked into subscription fees? Curious if anyone else has some thoughts.