With every fork you have yet another choice ... do something else.
If MySQL users will finally get p*sed off enough to be willing to fork it, they won't do it (most likely). They will just go to MariaDB (or hopefully PostgreSQL, but that's another issue).
When bzr users will be missing some functionality in bzr, will they spent all that effort to create a fork of bzr which they will then have to maintain, or rather sit down and read http://git-scm.com/book ... it is really not that scary (anymore?).