Linkshells don't make it automatically feasible. If the courier Linkshell doesn't get enough jobs to keep people interested, they are just going to neglect it until its effectively dead.
I agree that it should be a service offered by a group of people with other over-arching goals, so if demand for couriers is very low, they can still survive anyway.
I agree that it should be a service offered by a group of people with other over-arching goals, so if demand for couriers is very low, they can still survive anyway.