It’s expensive but probably less costly in the long run.
RTE overhead lines: 100000 km
RTE underground lines: 5400 km
https://www.rte-france.com/fr/document/statistiques-reseaux-...
If you are intending to distinguish between distribution and transmission, the lines in question in California are transmission lines, not distribution lines.
Math: 155147 miles * $750/foot
And an article https://www.desertsun.com/story/news/environment/2019/10/11/... confirming it's pretty much out of reach. $15,000 per household estimate (and 1,000 years!!).
And that's before you look at getting crews in place.
Then there's the question of monitoring, servicing, and repair after undergrounding.
It's definitely an option. But not a cheap or universally applicable one.
2-3 times more than per square foot price of building a luxury house in California, that number totally checks out!
An above-ground fault-crossing, with slace sufficient for maximum predicted ground movements (generally on the order of 5m or less) should generally address that.
A branch broke and landed on a power line, sparking a fire.
I'm curious what PG&E could do different, it sounds like the tree wasn't even very close to the power line, but wind pushed it.
Turning off power like this during wind-storms might be the permanent new normal.