Assuming that a trip to a Metro Bank branch will take around an hour, I’d still argue that this is competitive since an hour of your time is probably worth at least £5 - £10. Personally, I’d rather not have the hassle of having to visit a branch (& book an appointment, at least in some cases) either.
Either way, Monzo have been very clear about the fact that they won’t be setting up a branch network because as you hinted, the cost is considerable.
Jonas has mentioned that he’d like Monzo to implement this & we know that Monzo have been allocated enough virtual cards to enable it so hopefully this is only a temporary solution for Apple / Android Pay users, although obviously not all users will have access to that feature.