Many applications need a way to contact a user (security breach, password reset). If one only has a username and forgets the password, there’s no way to reverify the user.
Tough beans?
You have a business relationship between the company and a person. Whether that person remembers the password or not is immaterial to whether they have the legal right to anything they purchased in the app.
At this point the password is pointless, you might as well just use the email address. Or perhaps a distinct username and email address, but then there would probably be a “forgot username” workflow making that as pointless as the separate password.