On the other hand, for a startup-style outreach-focused project, substantial value comes from the case where the project outgrows the EA movement.
Yup, this is the classic "build your own market" vs "take over existing market." Companies that do really well usually end up expanding or creating a market. (For example, AirBnB allowed a lot more people than before to easily rent out their homes.) However, it's often easier to start by capturing / competing in a small market. So it seems best to me to start with a relatively small market, like EA, but have the brand and product aimed at a broader market. (Which is what I think you are saying.)