For 70 years there has been a continuing discovery of larger new ecosystems for new software to grow into, as well as many small ones. Before Open source became widely available, the software infrastructure (e.g., compilers, editors and libraries of algorithms) for these ecosystems had to be written by the pioneer developers who happened to find themselves in an unoccupied land.
Ecosystems may be hardware platforms (e.g., mainframes, minicomputers, microcomputers and mobile phones), software platforms (e.g., Microsoft Windows, and Android), or application domains (e.g., accounting and astronomy)
There are always a few developers building some infrastructure project out of interest, e.g., writing a compiler for their own or another language, or implementing an editor that suites them. When these projects are released, they have to compete against the established inhabitants of an ecosystem, along with other newly released software clamouring for attention.
New ecosystems have limited established software infrastructure, and may not yet have attracted many developers to work within them. In such ‘virgin’ ecosystems, something new and different faces less competition, giving it a higher probability of thriving and becoming established.
Building from scratch is time-consuming and expensive. Adapting existing software systems speeds things up and reduces costs; adaptation also has the benefit of significantly reducing the startup costs when recruiting developers, i.e., making it possible for experienced people to use the skills acquired while working in other ecosystems. By its general availability, Open source creates competition capable of reducing the likelihood that some newly created infrastructure software will become established in a ‘virgin’ ecosystem.
Open source not only reduces startup costs for those needing infrastructure for a new ecosystem, it also reduces ongoing maintenance costs (by spreading them over multiple ecosystems), and developer costs (by reducing the need to learn something different, which happened to be created by developers who built from scratch).
Some people will complain that Open source is reducing diversity (where diversity is viewed as unconditionally providing benefits). I would claim that reducing diversity in this case is a benefit. Inventing new ways of doing things based on the whims of those doing the invention is a vanity project. I have nothing against people investing their own resources on their own vanity projects, but let’s not pretend that the diversity generated by such projects is likely to provide benefits to others.
By providing the components needed to plug together a functioning infrastructure, Open source reduces the cost of ecosystem ‘invasion’ by software. The resources which might have been invested building infrastructure components can be directed to building higher level functionality.