Then keep using Git and let others that want to use Mercurial do so. To be honest, it's not clear what your argument is. You use Git so everyone else should use Git?
Almost everyone is using Git nowadays, so the rest should use Git also, yes, instead of wasting brain cycles, even worse huge efforts to support Mercurial instead of improving existing features in GitLab.
I'm not at all against progress, innovation or opinions, but I hate complexity, wasted effort and somebody forcing me to learn yet another tool just because...