I think a core problem is that merely changing the type and recompiling it is likely to cause other issues with code that is using time_t--for right or for wron

I think a core problem is that merely changing the type and recompiling it is likely to cause other issues with code that is using time_t--for right or for wrong--to store the result of subtracting two other time_t values; it also could easily be that a piece of software just sort of looks like it works after that edit but doesn't die to some less-used corner case. (And, FWIW: my expectation is that civilization will have collapsed because of the machines taking over, and the only thing that will save humanity is when they all keel over and die on February 7, 2106.)