Based on an inane interview question that was discussed here on Twitter: https://twitter.com/QuinnyPig/status/1265286980859908102
"chmod a-x $(which chmod)" not a particularly likely thing for a user to try to do directly, but it is conceivable for some sort of script to attempt it by accident because of a bug, and it would make the system inconvenient to recover. Since it's almost never a desirable operation, chmodding chmod itself could simply fail unless something like --force is supplied. The underlying safety logic is similar to that behind the existing "--(no-)preserve-root"