Unpopular option: I do not think that ethics needs to fall on the developers to maintain. I'll work in healthcare, government, finance, whatever. Innately, I dislike the idea of some industries or software being 'bad'.
We already try to prevent bugs with the mantra that if we build a better app, the world will just build a better user. And then the user messes everything up in new, exciting ways.
Reducing playtime per account would just make people multiaccount. Reducing EXP gains over time will just make people move on to another game after diminishing returns kicks in. Devices can introduce features to increase mindfulness, but nothing will stop a motivated person from getting another device if the first one starts stopping them.
Why should software try to take on that responsibility to block people from doing what they will do anyway?
Add legislation to help addiction not ban the things that cause addiction. Add features rather than try to cure additions through software. You can't, as the owner of a product, both want people to love and use your app while also try to make them not love it too much. If someone has addictive tendencies, they will have that in more aspects than just your app, so I don't believe it's on you to try solving or preventing that.
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