What's the point of closed source when any competitor can copy your entire startup at the click of a button? Going all-in on open source shows more of the confidence that you have in your brand, distribution, and maintenance/customer support ability, which is what matters in the end when the cost of building drops to zero. Sure it gets easier to copy a product, but that can help you to find issues and resolve them along with the copiers. The product itself is also becoming less and less important; brand identity and community is key. Security vulnerabilities are also somewhat more enticing/rewarding to find if you're unaware of what's running behind the scenes imo, so being closed source could be more of liability in that case.