While I'm also eagerly looking for an alternative to Git submodules (an initial pit of failure is that clones and checkouts aren't recursive by default, and in the long run it's annoying that branching/forking a module doesn't fork its submodules' upstreams), and the existing alternatives are poor (subtrees fail to resolve merge conflicts, and subrepos are unmaintained, fragile, and break when you rebase a clone/pull commit), I'm instantly put off by the slick commercialized sales-funnel design of your website.