All components eventually fail. To maintain a system operational, one needs to be able to fix or replace failed components. Replacing a high-tech component made

All components eventually fail. To maintain a system operational, one needs to be able to fix or replace failed components. Replacing a high-tech component made 50+ years ago (like a vacuum tube) can be prohibitively expensive even for military. At some point, it becomes cost efficient to replace the complete system, or at least large components, with modern equivalents.