Can't they design the tank system modularly to be replaceable? Like (also projecting how SpaceX etc are also making space cargo much cheaper) having a rocket carry payload that would replace the tank cartgridge with a new one, giving, say, another 5 years' worth of propeller.

I'm pretty much 100% sure NASA knows this better than me of course, but I'd love to see the reasoning behind planning to retire such an expensive project after a (relatively) short ~10 years.