What's the right policy solution to this? Genuinely asking.

The proximal problems pointed out here are:
- CA power transmission infrastructure is under maintained.
- CA has overgrown forests of which anything can be a catalyst for forest fires.

For whether the CA infrastructure was indeed under maintained, let's look at the costs and benefits:

The cost of maintaining the infrastructure more is that you'd have to inspect much more of the power infrastructure much more often. The twitter thread suggests that there were many orders of magnitude more quantity infrastructure like this, and this tower was under maintained by at least one order of magnitude. (To be honest, I'm not sure whether the tower lasting 100 years is evidence in favor of more preventative maintenance or less!)

Costs are made worse by the rising cost of infrastructure in the last many decades. Think about the cost of building a new subway in Manhattan and now think about replacing all the century-old high tension towers in CA.

The benefit of preventive maintenance is that you'd get fewer accidents, and perhaps less downtime caused by accidental failures. How much fewer fire accidents you'd get is uncertain -- the rest of the comments here do a good job of dissecting between the camp that says this could have permanently prevented all electrical-caused CA wildfires (maybe a few fires per decade?) and the camp that says this would have no long run impact on the number of disastrous fires because a lighting strike would have started the next fire.

Now it seems to me uncertain that the costs outweigh the benefits, but suppose it does, what's the best way to make PG&E do more maintenance?

Currently PG&E is profit-incentivized to not maintain infrastructure very much. The gov't could force them to establish a liability fund that they must contribute 50% of profits to until they have enough money in a trust fund to cover one bad wildfire. This policy would have direct cost effect on Californians -- after all PG&E is not a genie that you can legislate free money or performance from. PG&E would raise electricity rates by an amount equivalent to its current profit margin (not that much) and perform more fixing of infrastructure to meet the CA rule. This seems like probably a good route to take, as PG&E would then focus maintenance on liability reduction. The liability laws should be written carefully to incentivize PG&E to fix the right things. PG&E should only have a certain percentage liability per event, where the percentage correlates with "how many new wildfire did you cause / how fire-proof this region would have been if not for PG&E".

There are some other fixes too for the legislatively-minded. CA could just literally force PG&E to replace towers every 50 years, conduct 2x as many inspections etc. This would probably raise PG&E's costs which would get passed to CA utility payers. The disadvantage of this solution over the last paragraph is that the specific maintenance is by law, instead of at the discretion of PG&E's technicians. But if you think PG&E's management is poor to begin with, then that's not too bad.

But what if we just take more of it from executive's pay? Probably the executives will just leave and go elsewhere, in which case it becomes similar to the next solution: nationalizing PG&E. Now this could work. The engineers of HN in PG&E will have to tell me how much mismanagement is really going on inside PG&E compared to gov't owned utilities they've worked in in the past. The higher that ratio, the better the case for nationalization. The lower the ratio (e.g. if you feel the state of CA would run PG&E worse than today's management), then the less argument there is for nationalization.

Overall, the extra maintenance has to be funded by someone. Your pick of whether electricity prices go up, or whether fire-endangered towns need to make 2x property taxes, or whether CA residents have higher income tax rates.