> a blown fuse cannot be unblown in an economically feasible way

Isn't it like a bit of solder? Or is the problem that the solder would have to be a nanometer across?

efuses use electromigration whereby a higher than normal voltage causes metal to migrate away from the junction, not a wire getting burnt through like a regular sacrificial fuse.

If you wanted to patch a die the only real means is through a FIB (Focused Ion Beam) workstation, and even then you need the die structure to allow it (you have to mill your way into the chip and can't have any functional layers in between you and the patch you want to makey), and even when doable it would be an insanely expensive, time-consuming and likely fragile project.

A "fuse" in this context are transistors and bit-addressable RAM with no erase lines in chip, not physical fuses. They can't be unblown without a Star Trek replicator, or at least like a high powered electron microscope or something.