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.
I think the best we could hope for is maybe being able to drill into the liquid helium tank and refill it - which is the ultimate determinate of its useful lifetime (that and propellant, but I'm led to believe the liquid helium will run out first).
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.
There is no consumable liquid helium. Past space telescopes have used consumable coolant, but JWST has a closed refrigeration system to cool the one instrument that requires active cooling.
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.