Git had git-svn. I used it for a while to interface with a project that was still on subversion. So, I was able to gradually get my team off subversion. Subversion in turn had an easy migration from CVS and of course CVS was a bit painful to use so lots of people ended up migrating. I did such a migration once. It took a while to run but we got it done and kept our version history. We did not look back after that.

There probably is already some work on this but the path to success for pijul would be removing as many obstacles as possible between git and pijul and alternative systems such as mercurial. Make migrations easy. Make interfacing with git remotes easy for pijul remotes or interfacing with pijul users easy for git users.

Git is well entrenched of course with web UIs, IDE integrations, CI/CD support, etc. So there's more to replacing it than just writing a bunch of cli tools.

Neither of those had something like GitHub behind them.