For an app like this, open sourcing non-business-critical parts of the app a perfectly valid way to go about it, like they are doing with the templating library

For an app like this, open sourcing non-business-critical parts of the app a perfectly valid way to go about it, like they are doing with the templating library. You still get visibility, testing and the occasional bugfix from the community. Another possible way to go about it is open-core: Open source version, with more advanced features on a closed-source solution. Contributors to the open source version have to waive off some of their rights in order for this to work, though. There are tools that automate most of that these days.