I've been a Git user since 2007, but I disagree that we need only one VCS. Like another poster I've been through many variants over the years, from no VCS to us

I've been a Git user since 2007, but I disagree that we need only one VCS. Like another poster I've been through many variants over the years, from no VCS to using two copies (devel and stable) to SCCS to CVS and so on. CVS was fantastic compared to SCCS (because we had more than one developer, and we were already separately located), likewise Git is heaven compared to CVS. But even though I'm using Git for everything, we need Mercurial. And any future new VCS system with new ideas. Otherwise we won't know what's possible, where leaps can be achieved. Git didn't come out of nowhere, Linus put together the first version in just a week - he put down the fundament, the basic ideas that Git is built on. The reason he could do that was because he had used Bitkeeper previously (before Bitkeeper he refused to use any VCS at all), and his experience with Bitkeeper enabled Linus to come up with the Git fundamental principles (note that Git is different from Bitkeeper in important ways - particularly the core, where Bitkeeper built on (basically) SCCS and Git has a totally different way of tracking changes). Git (and Mercurial etc) are what they are precisly because there have been other version control systems before them.