There is a scenario where having the legacy automakers going electric can actually slow down the mission. Given that Tesla is able to make a better product from same or less battery than the legacy autos, diverting battery supply to build inferior products will slow down Tesla's production capability ramp for as long as the demands for good electric cars are not met yet.
i.e. given some battery supply capacity, Tesla is able to produce more of the better product vs less of the inferior product from legacy autos. For as long as there are unmet demand for tesla's product, letting legacy taking away some of the battery production capacity will slow down the mission overall.