Every success story and happy conversion to jj is evidence that hg should have won the DVCS war, but more importantly, that VCSes and their inherent merits and tradeoffs were always secondary to the social networks underpinning them. GitHub as a defacto monopoly really killed innovation in the VCS space, but also shifted the focus and attention elsewhere. That's why I don't think there'll be so much of a "post-git" without as much as a "post-github".
Does VCS-agnosticity actually work? It feels like a huge burden to migrate it everytime you want to have the tools from the innovation in your daily work. Also projects want to integrate project versions into each other and reference versions and identifiers are likely VCS specific. That's why I feel VCS monopolies actually has a lot of benefits.