The user wants the best product for their needs. If Basecamp only survives because its users don’t know about alternatives, then it’s not a very good business. This is actual competition: building the best product and letting the customers decide what they prefer. They already know about Basecamp in this scenario so seeing ads for other products will only let them make a more informed choice. The competitors, after all, are betting their advertising budget on the opinion that their product is better. It’s not free.