So release new products or paid updates, the way everyone did before subs.
> A one-time unlock front-loads 100% of revenue
Also known as "buying", which is a thing
> making all maintenance work have negative ROI unless the app experiences constant, consistent growth forever
Unless you release paid updates, which is what a number of developers do.
> Eventually that ceiling will be hit, and the app's revenue could drop to effectively zero, even if a million users count on it daily.
So don't put all your eggs in one basket. Have different products. Explore different things. You're essentially asking to be rewarded for stagnation here.
Subscriptions are about (1) being paid for work you didn't do, since you get paid whether you did anything or not; (2) disrespecting my choice as a consumer, since maybe I don't feel I need any updates and I don't want to pay for them, and (3) encouraging bloat, since there's more pressure to pack in needless features just so you can justify demanding payment every month.
You're basically saying "I need regular handouts because I couldn't make money otherwise." That's a problem with your business plan, and it doesn't justify demanding money for work you didn't do, or requiring me to pay for things I don't need.
I'll agree with you on one thing: years of free or artificially cheap software have conditioned people to not want to pay a lot for software. I always thought $0.99 per app was ridiculously low. I don't have a problem paying tens or even hundreds of dollars for good, powerful software, but I want to own it. If I'm going back through old files a decade or two from now, as a lot of us do, I want to be able to open them. And I don't want to have every one of my apps draining my bank account bit by bit.
If you want to offer a subscription for people who want every update you release, fine. At least they're getting something. But the fairer model is what a number of developers are doing: more money upfront, which includes a lifetime license and one year of updates (which is nice, but I would understand if they didn't include it).
There's a reason why so many people despise subscriptions, and why I doubt that model—while it seems to be the fad now—will ever be normalized. Most of us can't afford it, it denies us long-term access to our own content, and we know on a basic level that it's wrong.