1) I pay about 200 less a year for insurance for my Electra Glide Ultra Classic than I did for my old Nighthawk.
2) Let's compare apples to apples. Those considering both a Harley and a Japanese standard would likely not be considering a high priced big twin. They would be considering something like a sportster 1200 or 883 -- the latter is less expensive than many small bore metric cruisers. And, contrary to popular belief, is often available at MSRP. The 883 is actually less than a Nighthawk.
3) Customization. Again, compare apples to apples. Most metric cruisers will be customized as well. Often,these parts are comparable in price to those available for Harleys. Metric cruisers do not hold their value as well as Harleys. There may be a few exceptions, but not many.
4) We won't even talk about your financial assumptions that the extra cash would be invested, not spent on other things. Looking at the savings rate for Americans, the money would more than likely be spent on a nicer car or a surround sound system. People like myself choose a crappy car compared to what I could afford to ride the motorcycle of my choosing. In short, if I didn't ride a Harley (10-15,000 miles a year), I would probably not be driving a '95 Dodge Neon. People that don't ride their Harleys and buy them just to have would probably spend the money not spent on a Harley on golf or boats or something else. In that respect, the Harley might be a wiser investment.
5) The loan. Again, most people looking at an 18,000 dollar Harley would probably be comparison shopping with either BMW (also holds value well) or other, more expensive, metric cruisers. They would take out less in loans on the metric cruiser, but the difference in resale would probably balance the loss.
6) I agree with you in theory. It does make more financial sense to buy an inexpensive standard than one of the more expensive Harleys. But as I noted, people considering a Springer Softail are not likely to be satisfied with a Nighthawk. Most people citing resale value do not base their decision purely on that fact. Those that do buy harleys purely as investments are likely to resale them almost instantly with less than 1000 miles on the odometer. These people almost always make money. Those that buy them, spend $2000+ on accessories, ride it 5000 miles over a few years then try to sell it, are probably not using the resale argument as their primary defense for buying the motorcyle. Either that or they are dumb. At the very least, those that buy it "for investment" or not going to keep it for the 10 years you build into your argument.
7) Ride what you like. I ride a Harley. Used to ride a nighthawk. The Harley gives me infinitely more pleasure. If the standard makes you happy, go for it.