There are two key reasons why spending a disproportionate amount on advertising targeted at existing customers doesn’t make sense:

1) You don’t need to. Customers will experience other brand touch points because they are customers, including the user experience itself which tends to have a much bigger influence on future purchasing than advertising.

2) Customer churn is impossible to avoid. Even if you deliver a competitive user experience, you will inevitably experience lots of customer churn. Customers enter and exit the market all the time as their circumstances change. Also, your competitors may sometimes offer a deal that's too good for your current customers to resist.

In order to grow, you need to focus on acquisition, as Ehrenberg Bass Institute has illustrated so effectively.