[cperciva]

I wonder how much demand there really is for it. Many (Japanese) people I know would rather stick to bank transfers

Even if that's correct... you're talking about how Japanese people like to pay for things. Stripe entering Japan isn't about allowing people in Japan to pay for things using credit cards; it's about allowing people in Japan to get paid using credit cards.

Sure, Japanese companies will have a higher proportion of Japanese customers, but this will be important if they have any customers from outside of Japan.