The majority of open source software development is "implementing something that already exists", but with improvements, such as for specific use cases and constraints (like the original NES emulator) or by making it more performant. That's how the ecosystem mutates and grows, and it's worked well for decades.

>The majority of open source software development is "implementing something that already exists"

I don't think open office/libre office etc have access to the source code for MS office and if they did MS would be on them like a rash.