If you have a healthy churn rate (say 5% or less) and you're getting some residual number of signups every month through SEO or some other passive channel, then

If you have a healthy churn rate (say 5% or less) and you're getting some residual number of signups every month through SEO or some other passive channel, then theoretically you could coast for a long time. The obvious downside being that your competitors may eat your lunch.