If Reddit starts charging for its API like Twitter is doing, Reddit can expect a swift exodus of its most dedicated users and a decline in the quality of its content. Nobody wants to suffer a bad user experience, and nobody wants to pay for the privilege of contributing content to a for-profit company. In the long run, the only social networks that will have a durable future are the ones that allow alternative third-party clients. All of the others will inevitably devolve into a mess of low-quality content and increasingly intrusive advertising.
RedReader and other third-party clients made Reddit tolerable. Take that away, and many users will stop tolerating Reddit.