Not sure if someone covered this already, but figured I would give my two cents since I worked in the industry. Usually the hardest part of electrical work is taking a circuit off-line long enough to fix an issue like this. The toughest projects were upgrading lines where customers are fed radially (one circuit). In more densely populated areas customers are fed by two or more lines, allowing the utility to schedule an outage and do repairs or upgrades. Utilities are judged on metrics that track outage time (I believe its called SAIDI), and they will do everything they can to use those lines until the end.
Anytime a utility decides to upgrade their system they have to think about a couple things. First they need money and that requires them to secure funding from their regulatory body, which will determine if they can raise the cost of electricity for the customers. Next they have to put proposals for work out for contractors. Normally utilities are not staffed to handle major projects so they might hire a 3rd party firm to manage the work. The utility must schedule outages, which requires planners to balance the needs for maintenance with the need for long-term upgrades. If they are going to replace lines, they might as well upgrade to a higher voltage, which may require the replacement of equipment in substations, which can take years depending on the size of the sub. Time of year must be taken into consideration because electrical usage in the summer is higher, and taking one circuit out add load to other circuits and could cause failures.
You might work through all these scenarios and find yourself at the mercy of nature due to endangered animals. Birds will nest on powerlines, and prevent any maintenance to go on for months. Even if you want to do hot-work via helicopter, you are prevented from encroaching the bird by a buffer to prevent disturbing the bird. I don't know how big PG&E's system is, or how much they are willing to spend/are spending now on maintenance but this is an industry-wide problem that some utilities are handling better than others. Maybe wireless electrical transmission will solve this?