It's a hard limit due needing fuel to maintain its orbit. It's in a lagrange point (https://en.wikipedia.org/wiki/Lagrange_point) which requires occasional orbital corrections.
-ish. They have no firm plans for servicing it, but it does have a docking adapter and the fuel/coolant connections are designed to be usable in space.
Basically, because there is no reasonable way to service something in L2, they can't really plan for it, but it's expensive enough that they made sure there is the capability if someone in the future would, say, build a spaceship that is orbitally refuelable and designed so it can take crew that far out.
Well it's not strictly a hard limit but it's currently planned to be a hard limit. If SpaceX can pull off even a fraction of what they claim with Starship, it's not unrealistic to think that it'd be financially viable to attempt a refuelling of the JWST.