I love what Purism is doing with its Librem phone, but I do not believe native Linux apps are a viable strategy. If Microsoft could not convince major app makers to support Windows Phone with its incredible UWP developer experience, I do not have hope for native Linux apps.
The only pragmatic option for any hopeful contender challenging the Android—iOS duopoly is to bet on the Web.
You might say, “I don’t need official apps. Those services have public APIs and I can build a Linux client.” Except for Twitter, I haven’t used a third-party app better than the official app. And many companies—like Fitbit—explicitly prohibit recreation of primary experiences.