Erich.. you're extrapolating some things I did not say from things I did say.
FACT: Belgian posters have dropped in value steadily since fleaBay. For the most part, Belgian posterssold for much more money 10 & 20 years ago than they do today. The reason being of course that every dealer who has Belgian posters keeps offering them ad-nauseum in quantities the market can no longer absorb (since maybe 2000-2002) and as a result, the prices have crashed on 95%+ of Belgian posters and maybe 99% as you really have to get to great post-war titles for any value support. Of course, pre-war posters are a different category entirely and value has been & will continue to be retained because they are very rare for almost any title.
Japanese posters are newer to the market for the majority of collectors who now collect them. Aside from Godzilla posters and the occasional Kurasawa, I don't recall seeing them too often before maybe 6-7 years ago and since then we have seen quite alot of them as they are sold regularly by Bruce, Heritage and by dealers on fleaBay. No doubt for a certain number of years, presuming the supply is not really any greater than it is currently, prices will remain constant.
However, if a number of Japanese dealers begins to flood the market by keeping constant quantities of posters on fleaBay and they lower prices to adjust to sales quantities - then values will drop just as they did with Belgian posters. It's a simple economic equation. Japanese poster dealers I think have exhibited much more control or discipline in this way than the sellers of Belgians did.
Now where you extrapolate is that you think I'm saying there is a huge warehouse supply just waiting to be purged into the market.. I don't know if this is or is not the case, but by all measures of printing all over the world, the Japanese have always been very prodigious printers of paper and also like places all over the world, there have been warehouses of paper just waiting to be found and distributed. I can't figure that Japan would be any different in this regard. But as I said, the Japanese dealers have been very good stewards of the material, keeping prices up so they only need to sell a minimum to achieve whatever price targets they have. If they have a warehouse of material - more power to them. However if they do have a warehouse of material and they decide to forget discipline & sell like the Belgian posters were, the prices will plummet like the hammer Newton dropped from his window.