A big problem with retrofitting above ground to below ground, especially in California, is the environmentalists aren't going to let it happen in many places it needs to. Especially when it comes to crossing creeks and dry washes, additional underground cable burying is going to cause a lot of land disturbance. And you're not just burying power in many of these places. Telephone and cable are strung along those same poles. Also many times poles to individual homes can cross sensitive areas because they are low impact, especially in very rocky locations.
Unregulated growth wherever people want to build isn't going to be an affordable problem, no matter how much money people think California has.
Unregulated growth wherever people want to build isn't going to be an affordable problem, no matter how much money people think California has.