Everywhere is different, but generally front-ends/clients are less scary to rip up, replace, and completely re-write.

If you're going for the unsackable by dint of arcane knowledge in the long lost technology from another age you want to be right in the guts of things. Something close to the core where everyone is worried that one mistake will cause problems that'll ripple out in all directions.

Just like those kernel hackers mentioned above, or the people writing business operation code in Cobol on a main-frame.