Countries a, b, c already can reuse their code/don’t duplicate effort. Others will join for sure. They’ll also get free code/bug fixes from people who want to contribute. People do it for many reasons, just listing you’re contributor to gov project can look nice on cv. No tax money is paid for that. If other countries want to use but not contribute back - so what? I don’t think it ever bothered anybody that their open source lib/whatever was used by people who don’t contribute back to it or by people who themselves don’t have open source projects.