I don't think any of these arguments are good enough to not simply put git submodules as yet another option to consider. Probably not your first choice but it

I don't think any of these arguments are good enough to not simply put git submodules as yet another option to consider. Probably not your first choice but it could be a perfectly acceptable one (think components). Git subtrees are definitely not a replacement for what submodules can do. It's actually closer to monorepo. Package systems are absolutely more robust but could be more painful depending on context. Git submodules, if correctly understood (which is not trivial, even in git terms) can be just the right tool for a certain scenario. Qt has used them quite well but it's definitely not as friendly to newcomers as it is to "core Devs"