Ask HN: Why Open-Source for Products/Startups?

I work with investors and entrepreneurs - and I often suggest starting with an "open-source by default" mindset, including building a community around an open product.

Some people don't know about open-source, and mention "what if someone copies my idea" and "how can I generate income if its free". I'd love to be able to send them (1) a good high level overview of the What / Why of open-source, and (2) Point to successful examples of open-source commercially successful products & services.

Some ones that come to mind are: RedHat / RHEL, Supabase

Thanks!