You don't need copyleft licenses to have free software. There is plenty of code that is written that is not a key differentiator, that people will still open so

You don't need copyleft licenses to have free software. There is plenty of code that is written that is not a key differentiator, that people will still open source. Parent mentions Goldman Sachs; their trading platform is not something they want to sell, nor want to give away. Their build system, their UI component library, their numeric libraries, etc, could all be things they don't feel give them a material advantage over their competitors, and so it makes sense to open source ('many eyes make any bug shallow' style of thing). These libraries don't need to be copyleft. If someone else wants to extend them and turn them into their secret sauce, who cares? They lose the benefit of it being open source (those many eyes), but if keeping their modifications closed enables a business model that people want to pay them for, they're generating value. And that's something that copyleft -wouldn't- allow, but a permissive open source license (such as MIT) would.