While nothing is irreplaceable, the cost of replacement is an issue. You could set up a wafer fab up or re-design the board the component sits on or replace the module or replace the system.
I suppose you could use some parts found in the back of a drawer with no documentation, though.
Concorde had this massive problem; in terms of avionics she was streets ahead of anything else flying ANYWHERE IN THE WORLD at the time. She had the worlds very first digital control system (Decision to change design of this system from analog to digital taken in OCT 1970; not actually flying in the British Pre-Production A/C until 1973). When parts for this system (The AICS) started to become obsolescent in the 1990's, replacement IC's had to be specially designed, manufactured and certificated. (The re-designed component boards worked like charm and were infinitely more reliable than their predecessors). I guess in a way Concorde was like a flying electronics museum; from the days of analog all the way to the digital age. (Also the finest aircraft ever built).
