The problem with your question is that you don't understand retention (nor the author of the post explain it well). Retention in theory is not that I buy from your service every day. Retention is that, whenever I need your type of service, you are my first choice.

Obviously it is very difficult to measure this retention, so you measure returning users. But you have to understand your market.

This said, paying for keeping users is not unheard of. Discounts for loyal customer, targeted email for new purchase etc etc...

Usually this value is factored in the LTV (Life Time Value of a customer) and you could play with it to optimize the LTV versus the cost of acquisition (Cost of retention should be lower of cost of acquisition, if it is not, look at your product/service because it sucks).

But from your original question, it seems you have a "one-off" type of service. Think about wedding planners. Your problem is that if you acquire customers faster than new customer are born, you are in trouble. Otherwise it is a sustainable business, just take care because you have an upper bound on your growth and your cost of acquisition should be lower than the profit you get in every transaction (excluded acquisition).