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So, why do manufacturers put all this redundancy into the systems?
Isn't the reason for this that it has proven to be cheaper to carry the "spare parts" along in the aircraft, all fixed and wired into its proper place, ready to go, rather than have an engineer work the aircraft during todays thirty minute turnarounds? Thus, should anything fail, this "fail operational" system redundancy allows us to depart after consulting the books, confirming we are indeed safe to depart with one generator/one flight control computer/one radio receiiver INOP?
Why fix one of three COM sets and be proud of the resulting three hour delay because it was not the COM set itself that had failed, but the wiring someplace unreachable? Just refer to the MEL, which had a lot of effort put into by people who had all the time in the world to think about all those remote possibilities thar might occur, and when it said it is OK to depart, off you go.
The reason we today carry more devices of the same kind is not that modern equipment fails more often, but rather to be able to schedule time consuming work when the aircraft rotation allows.
Yes, the tristar was a great aircraft back then, but there is a reason why airlines today operate the aircraft that we see on todays airports.