Only thing I can add to this is that some restaurants will say they take cards, but they'll have a minimum amount of 4-5000jpy to be able to charge your bill, so it really does depend on what you plan to do and where to go. Entrance fees to cultural sites or museums can be cash only as well.
It's becoming more cashless, but as noted above, some of the smaller privately owned restaurants are cash only, even in touristy places like Asakusa.
I bring about 100k jpy in cash with me as an initial fund, but I tend to do cash only things like going to the arcade or the horse races during my time in Tokyo.