Engagement systems are built on the assumption that motivation stays fairly constant. You set up the program, people join, and if the design is good enough, they keep showing up.
That's not how motivation works.
Recent habit research puts it clearly: motivation fluctuates by nature. What separates systems that retain people from systems that lose them isn't the strength of the initial motivation. It's whether the design accounts for the low-motivation moments.
A streak mechanic pressures people during those moments. A well-designed system reduces barriers during them.
The practical question isn't "how do we keep people motivated?" It's "what does our system ask of someone on their worst day, and is that realistic?"
Most drop-off happens not because people stopped caring. It happens because the system expected them to show up at full capacity when they couldn't.
This is really fascinating. Because it also says that you need a design that holds up during high motivation but reduces barriers during low motivation.