Comment by bityard

It's MUCH more expensive to design and build an aircraft from scratch than it is to repurpose and maintain an existing design that fits the requirements. The major cost sinks are not even the design and manufacturing, it's all of the testing, certification, training, documentation, maintenance planning, and so on.

Plus, it's very likely that this plane is not an ancient as you think. New airframes are more efficient aerodynamically, weigh less, and offer more capabilities but depending on the role, those may not be huge advantages. Nearly everything ELSE on a typical airplane can be upgraded to modern standards. I haven't checked Wikipedia, but I highly doubt NASA's WB-57s are still running the original 1953 engines and avionics, for example.