Some customers will churn, some will stay, Bending Spoons are the masters of this model so will have made an assumption on how revenue will change across the next 5-10 years+, but I would assume that they aren't forecasting extreme growth, and instead are calculating that net profit can be changed from c$30m to c$139m within existing revenue, so if they can keep revenue at/near current levels without growth, they can end up with a much more profitable business.
Bear in mind that same revenue doesn't necessarily mean the same number of customers - it can also mean raising prices and having less customers. Bending Spoons might estimate that if they double prices, half their customers might leave - this would still be BRILLIANT for profit, as while revenue would stay the same, some costs would half, and thus profit might jump from c$140m to c$250m based on some napkin math!