If its just “one line of code”, why not make a contribution? Doesn’t sound to difficult. 
If the password is not strong enough, it was the deliberate choice of the user. Cryptomator can only motivate the user to use a strong password. An additional thought: If the user chooses a weak password, why should they care about the masterkey location?
That said, as @overheadhunter stated in Rename masterkey file to prevent attackers from deleting it - #8 by overheadhunter, it is a good thing to let the user decide, but this needs a new vault format. And one does not easily introduce a new format! (as it can be seen from the switch from format 6 to 7^^)