Maybe if we didn't have the absurd excuse of "the tyrant is evil, but at least he'll die someday" we would be more inclined to do something, instead of sheepishly obeying and letting time pass.
The tyrant can build a fortress to slaughter 99.999999% of attackers and be safe until a million men or more oppose him at once, but no stronghold can protect against death itself. And as long as he fears death, the fortress can be invaded in sufficient numbers through the personal physician's private entrance.
Besides that, if I and the tyrant were both immortal, I think I would still prefer to wait, at least until my odds of success were better.
