well except that there is no FOSS license that requires you to submit your changes upstream. so the license argument is not going to be valid in most cases. GPL only require you to share with users, so any in-house use of software does also not require you to share the code with anyone outside. AGPL might trigger sharing if the software is used in a website, but also only with users of the website, not with upstream.

only the maintenance argument holds, but that is a trade-off, not a legal requirement.