As I see it, the government's real pay-off isn't Tesla paying back it's loans, but rather is the increase in energy policy flexibility that is gained if EVs become popular. If it's easier to provide energy security, that reduces military burden and also reduces our need to be involved in some unfriendly parts of the world.

This payoff can't be captured by a private investor, so the government has incentive to invest earlier (and at higher risk levels) than private investors.