I used a WeWork credit to work remotely while in Tokyo around a year ago, and the place was packed. Every common seat was filled and I even felt like I was getting in the way because of how often people were going around me to banter or talk shop. Also the internal WeWork forums in Japan seemed to be a genuine social network with heavy Linked-In style participation.

I'm sure some of the hustle and bustle is due to the novelty, much like how WeWork was when it first launched in the US. However, you're right that there is a real lack of flexible month-to-month space in Japan so that may be a lasting advantage.