There will always be inactive customers, but if your customer growth month-over-month is not greater than your % of inactive users (the inverse of % active users), then you will stall and enter decline.
For example: if 80% of current customers login on a regular basis (monthly, daily), and you're growing at 10% per month new users, you will actually decline at some point because your 20% inactive users is greater than 10% new customers. It might take some time, but at some point customer churn will outpace new customer growth.
So you have to measure new customer acquisition as a % of current installed base, total % of actives (measured on a logical period relevant for your solution), and total churn % (non-renewals divided by renewals). If the churn measure is measured differently you can think of it as the renewal rate because you want to measure retention vs. acquisition.
To simplify: if acquisition is not greater than retention, then you have problems. Retention is dependent on active use with some factor and with some delay.