Flying is easy when everything goes right. We train for the things that almost never happen. It almost never happens that transport category airplanes lose both engines at low altitude; but it happened last month.
Some of the things that almost never happen attract the attention of the FAA or other regulatory bodies. The FAA collects some of this information in the form of Safety Alerts for Operators, or SAFOs. They are on the here. The target audience is operators of transport category airplanes, but some of the information applies to everyone.
For example, SAFO 09001 addresses "Effects of Aircraft Electrical Faults Resulting in Main Battery Depletion," following an incident involving an airliner. They had an electrical failure, and rather quickly ran out of battery power. They decided to land before things got worse, and did so safely. A little time reading NTSB reports will convince you that airline pilots handle these events well.
But what about our flying? Private pilots are taught to handle electrical problems, and instrument-rated pilots are taught again. It seems to me that most pilots retain this information pretty well; I can't recall anyone getting a BFR who didn't have a reasonable answer to "What do you do if your alternator fails?" I have had a few alternator failures, and am not bragging when I say that I handled them easily.
But let's think this problem through again. There you are in your high-performance single, traveling from Boise to Portland. You use one of the online flight planners, which sends you along V500 to Kimberly. It's 166 nautical miles from Boise to Kimberly, and then another 112 from Kimberly to HARZL.
What's the issue? You can probably carry more than enough fuel (today that route works out to about 3.5 hours in my club's Cherokee Six, which can carry an effectively infinite amount of fuel). But what if that single alternator fails? I would go through the checklist and reduce the electrical load. If VFR, I could continue using a watch and a sectional (I have flown from eastern South Dakota, all the way across Wyoming, and well into Idaho without any electrical power, and another time from Idaho to the California coast and back. I thought it was fun.)
But what if it's IFR?
Looking over the enroute chart, there is nothing between Ontario, Oregon (which is essentially a suburb of Boise) and the Portland area that has an instrument approach that the Six is equipped for. I could end up flying for hours, in the clouds, across the Cascades, with no way to navigate.
So I need to change my thinking from "I didn't plan on that happening" to "I planned on it happening." So instead of going over Kimberly, I go over Baker, Pendleton, and Klickitat, basically following the Columbia River. There are many more alternatives, and it only adds 15 minutes to the trip.
I remember one night in the King Air, Boise to San Jose. I had one of the less-experienced pilots along. The passengers were strangers. We were cruising along in clear night air at FL280, but a large area of high pressure covered the northwest, and ceilings were low.
"Brian," I asked, "check Reno weather, please." Visibility 1/4 mile, vertical visibility 0. It was the same with Lovelock, and Battle Mountain, and Winnemucca. The fog didn't go very high: we could see the Hilton in Reno as we flew overhead, sticking out of the fog. But there was no way to land there. "I don't like this," I said.
"What are you worried about?" he asked, "We're at flight level 280. What could go wrong?"
I gave him a long list. Engine failure. Pressurization failure. Bleed air failure. Passenger medical emergency. Who knew what else?
A truly risk-averse person would make a personal rule "always have an alternative airport within 20 minutes flying time," kind of like the old overwater rules. In most parts of the USA this is easy; where I live it is not. In fact, where I live it is sometimes impossible. So we have to accept that "almost never" is almost never. The risk can be minimized, but never completely removed.