Unfortunately, even if we do develop the capability to get astronauts out there, the Webb is constructed with a lot of adhesive and layered components and is basically impossible to service in space.
Probably a moot point though: by the time we can get people out to do such an operation, I suspect a reduction in launch costs might lead to a much cheaper space telescope you could just drop off instead.
If all goes as planned, the limiting factor is propellant. And the propellant tanks have the ports for on-orbit refueling, in case there is ever a desire to develop a robotic servicing mission for the task.