inflation reduces its purchasing power
Yes, certainly. You'd have to adjust the figures in the chart for inflation, of course.
A better proxy is to look at savings as a percentage of income:
As you can see, that mostly recovered in the 2008 credit crunch after a period of decline. Seems like spending went up after covid, which is probably the effect we see of increased inflation/energy costs/commodity prices etc (which of course is all conflated into one complex). But the 4% the US is at now is fairly typical for the past 2 decades. Again, no signs of Americans getting poorer and not being able to afford Nikons from Japan. I stick with my earlier argument that the high prices we see for camera equipment are due to demand - and, as you pointed out of course, inflation. Can't compare today's prices to those we paid 15 years ago, for several reasons!
On the topic of Japanese sellers, you're also correct we pay significant taxes on imports. I always take into account that my purchase from Japan will cost me around 40-50% of the agreed upon price. This accounts for shipping, taxes, inspection costs etc. The few items I bought from Japan were always as described, in good order, transactions well tended to - nothing but positive things to say about it, really. I've bought items from Europe, the US and China with much more iffy aspects, such as defective items where defects were not or not clearly described, items not arriving due to simply never having been dispatched, items being sent only after weeks or even months of delays, poor packaging, etc.
No, can't say that there's a particular problem about Japanese sellers in my experience. If anything, quite the opposite!