As for let's go not doing as well in Japan compared to WW, it has to do with the fact that in Japan the audience for Pokemon is already pretty much maxed out and there wasn't a Max Exodus of players after gold and silver like in the west. Ergo the people who have nostalgic feeling towards Pokemon were mostly still playing black and white and XY. Ergo, thouse ppl are actually waiting for gen 8 to come much like a lot of is more hardcore fans are.
Go didn't have as much of an impact cause the people who would be interested were already playing mainline.
They better have like 2 milion units on shelves in Japan when gen 8 hits cause I think the switch might sell at least 1 mil during gen 8 launch month.