"if a technology like Dopamine can actually encourage retention on apps aimed at self-improvement, that’s an inarguably good thing."
No, it absolutely is not. Some of most horrifying abuses in the history of the human race have occurred under the guise of "improving" other people for their own good.
The men and women who write your software control the boundary conditions for your experiences there. They control what and how you can think, feel, and believe in that experience. When Twitter doubles the numbers of characters you can tweet, they're making a decision about the quality and size of thoughts people can express - and in turn what depth of dialogue we have there.
Analogously, when someone designs a diet app, they're creating the boundary conditions of a new behavior pattern for users. Their use of Dopamine aligns well with their end user's goals: in the same way that you hold friends close because you know they rub off on you, so too should we think about our software. Having an app on your phone you chose to let close to your mind and behavior - to be a new venue of your thought and future self - is a decision about who you want to become. And if that app can use good design, a good interface, and persuasive tech to help you achieve your health or diet goals: you win.
That's an alignment of what tech wants and what humans want.
You can't trust people to make that decision themselves. If you keep an app close because of the skinner-esque variable reward ratio I'd argue that's not a conscious choice.
Letting a person choose 140 or 280 characters (which originally was done for technical reasons -- 140 octets was the maximum length of a classic SMS message) is an entirely different thing from cold-bloodedly psychologically manipulating them.
First, those apps are not your customers and you aren't fooling anyone.
Even if they were today, your VCs and future investors will not allow you to turn down profits and only sell to health or self-improvement apps.
If your product is successful, you have a fiduciary duty. If you fail in that duty, you will either be removed, or potentially even sued. And then it will be too late, because you already built it, and someone else will have taken the wheel. And they will maximize it, inevitably weaponizing it against the population.
There are better things you can be working on. Just walk away. Humanity does not need this.
No, it absolutely is not. Some of most horrifying abuses in the history of the human race have occurred under the guise of "improving" other people for their own good.