For a solo founder, the bottom line is neither traction, recurring revenue, nor monthly active users. The bottom line is emotional health/fitness.
For example, as a solo creator, anytime I resolve my single-biggest source of stress I get in a flow producing my best work. I'm more open to looking at problems from new angles.
Without the typical external structures, emotional fitness dictates everything: ability to make decisions, ability to forecast, willingness to learn a critical new skill/approach, persistence through challenges, willingness to take risks, etc. Basically, if it matters, being mentally/emotionally healthy fosters it.
I don't know if it's a saying, but it should be: Great products come from great teams, whether that's a team of 1, 10, or 1000 people.