More than any individual feature of the iPhone, it was the App Store that was revolutionary. The homogeneity of one phone used by so many made it compelling to develop. Development complexity got progressively less difficult.
It may be some time before we see an equivalent market to apps for iOS or Android. Yes, Alexa skills and Actions on Google are markets, until more users have combined screen and voice interfaces, the market won’t be as compelling. Another barrier is that both these companies need to allow the steaming of audio and not just entities and intents to third parties to allow for interesting applications to be developed.
In order to bypass the restrictions of the platform providers, some companies will develop their own hardware that then layer on top the assistants like Alexa and Google Assistant. If the interaction is delightful, the there’s a chance organic growth will be high. However, the adoption won’t be geometric as quickly as other platforms because of the requirement to move around atoms.