Look at React - if it had never been open sourced, Facebook might still be using it, but it wouldn't be the same thing it is today. For one, basically all of th

Look at React - if it had never been open sourced, Facebook might still be using it, but it wouldn't be the same thing it is today. For one, basically all of the current excellent React team probably wouldn't be working at Facebook. And it would be far, far harder for Facebook to recruit engineers for product teams who were proficient in React. Since it is open source and very popular, the odds of a browser introducing a change that unfixably hurts React's performance is now very low. Et cetera. The cost of maintaining an open source project is real, but when it is a world-class piece of infrastructure, open sourcing it helps keep it world-class.