Though this is oft-repeated: nothing is "free". Pocket plans to make money off you somehow, whether directly or indirectly. Instapaper is simply more up-front about how and when you will pay.
> Instapaper is simply more up-front about how and when you will pay.
Whatever Pocket ends up doing, nothing's preventing Instapaper from doing it too.
They may not need to, but as we've seen in the past, you can't assume that that one-time purchase means you're now the customer again (and not the product).
The difference, as far as I see it, is that Pocket makes being free a selling point, whereas Instapaper users are used to paying for features. Instapaper can continue to exist without substantially shifting business models for the foreseeable future - Pocket can't.