It seems like most everything digital is replaceable with FPGA and PAL chips.

Most of the new DIY/homebuilt versions of old hardware (Apple I, ALTAIR, Amiga) that aren't faithful replicas, use FPGA and the like to replace many components and circuit board traces all at once. At some point, a Mobo replacement for old compact macs might be just a chip (or two) on a board with the proper connectors wired to it.

Bit rot to the programmed silicon chips would be my guess as to the biggest threat, but I'm no engineer nor fortuneteller.