Many end up as professional copilots who can be very pleasant colleagues but are a liability when the operation strays outside their rehearsed little box.
For many years in my former company we had co-pilots who had failed command courses and were permanent co-pilots. After many years of some quite marginal people being allowed to continue their careers a program was put in place called Managing Operational Performance. Thse who find themselves in that program either meet the required standards after further training or they are eased out of the company. The days of the not very competent being allowed to continue have long gone as I understand it.