ak67
Saying that Meta "reinvented CVS" means you either know very little about source control or you are just being purposely misleading. Neither is a very good look.
The needs of a large internal code base are often quite different than what open source projects need.
Anther issue is that tools like Git and Mercurial need to be compatible with positively ancient repositories and can't really break backwards compatibility whereas within Meta, it is easier to move faster and deal with breaking format changes since developers's checked out repos on their laptops can be silently upgraded since the machines are well managed with Chef and can be upgraded behind the scenes. In the outside world, you can't assume people will be running the latest version and can't upgrade them.
Finally, Google is also moving away from Mercurial. Perhaps the JJ developers want to respond why (and check out Jujitsu -- it's a novel and very interesting system y'all should check out).
The team working on Sapling has been planning this release for a very long time and will continue working on Sapling to support our internal engineering efforts.