As a community member since 2005, a former staff member of the Wikimedia India chapter and also the Wikimedia Foundation, I strongly believe the number of volunteers in Wikimedia projects can be increased as below:
The interface is not easy to edit on mobile phones and the majority of the new generation users use only mobile phones. We need to heavily invest in improving our technology and user interface. We cannot expect the users to go through a steep learning curve to contribute to us just because we are a non-profit project. We need to separate the workflows for content collaboration and distribution. This is even more relevant for projects like Wiktionary, Wikisource, and Wikibooks which can have wider adoption if they have modern apps.
Wikipedia’s vision says: "Imagine a world in which every single person on the planet is given free access to the sum of all human knowledge. That's what we're doing."
Free knowledge doesn’t need to be produced only through free labour. The current volunteer-driven model only works in most developed countries as we can clearly see 20 years after we started Wikipedia. We need to explore different models to encourage and empower the volunteers. Even simple steps like providing free laptops, reimbursing the internet bills can go a long way in empowering the editors as we can see from the results of Project Tiger, a project which I initiated as a staff of the Wikimedia Foundation.