Who says they'd have to? If you shift ownership to counties or cities serviced by PG&E, San Diego wouldn't have to pay taxes. Think more deeply about solutions to the problem.
I live in California bud, I know how the municipalities operate here (very poorly). I'm thinking much more deeply than you are. If I own the only powerlines in town, one of the rare true monopolies in America that will keep my family rich for generations, I'm not trading that for a one-time payoff.
IIRC SDG&E said that they'd want to sell their grid if PG&E and SCE were sold to the state. They may just decide this is a good time to sell out and transfer the risks to the public.
The transmission line monopoly is such a cash cow that really doesn't make a lot of sense. For the life of me I can't figure out why utility poles don't have wireless communication such that when one node loses power, the previous one kills power to the downed line (which in theory should happen faster than it takes for the line to fall to the ground and start a fire). Once those lines are modernized, the potential for profit is enormous. It doesn't make any financial sense to give up such a rare monopoly.