Perforce is more the IBM of VCS. Older than it has any right to be. Has quiet "dark matter" support contracts with a lot of companies you wouldn't think need Perforce, but they've been using it for long enough they aren't going to change. Some of those support contracts included complicated forks and homegrown solutions that are so sunk cost as to be nearly black holes (and sometimes so different from baseline Perforce as to be evolutionarily different species).