"Hey Don, where ya been? Grin"

I've been in here every day. But sometimes I have nothing to say.

"This is an interesting theory... any explanation for why the battery's life is so dramatically shortened?"

There are known reasons why a lead-acid battery's life is shortened by deep discharges. But why it's exactly six months after is somewhat of a mystery to me. The same thing happens if such a battery is under water for a while. For an example, an uncovered boat where the drain plug wasn't removed and then a lot of rain. In both of above cases, it increases the internal corrosion and I assume it takes six months for enough internal corrosion to damage the plates inside the battery. The corrosion can short out the same plates in the battery, when enough of it gets between any two plates. It's best to keep lead acid batteries dry on the outside and well charged.

"Deep Cycle" batteries that are used in larger boats and RVs that are NOT used to start engines (there's a separate battery for starting) are larger and have the plates spaced out more. These are much more forgiving, mainly because it's harder for an equal amount of garbage to short out the plates of the battery. These batteries are more expensive, larger & heavier but not good for starting engines. The amp hours of batteries is NOT consistent at different discharge rates. IOW, with a lead acid battery, a 100 AH might work well for one 100 amps for an hour, but not 100 hours at one amp or vice versa. The Deep Cycle batteries are made to run lighter loads for much longer times. And engine starting battery is the opposite.

Because of this, the battery manufacturer will always rate the AH rating at it's best point. However, there is a standard called "reserve capacity" that will tell you the AH rating for what the battery was designed for. These ratings are more common with deep cycle batteries. Engine starting batteries usually only rate the CCA which tells us how much current can be drawn from the battery at a certain low temperature. This is usually all we need to know for a car's engine starting battery.

"As an aside: Why do you think small motorcycle batteries are more expensive than car batteries? Economies of scale? "

IMO, for the same reason all motorcycle parts are more expensive. Not as many identical parts made when compared to the same part for a car.