What would it take to unseat Make? Make is installed everywhere, so it is really hard for me to justify leaning on new tooling. Make is ok, but it has enough deficiencies that I longingly look at tools like this.
I definitely don't think Just will ever unseat Make. Just doesn't have file-based dependencies, so it's not a build system, just a command runner.
As far as unseating Make as a command runner, I think that might just take Just being available in more places, since one of the main advantages of Make that many users cite is that it's available everywhere. Just is already available in a lot of package repos, but not all of them. Finally packaging Just for Debian[0] would help a lot.