the math sounds right, but actual numbers are not the same
when i get hit for 20k, how do you expect me to use just gear to get that back? with 1000 life per hit (which is alot) it still takes well over 10 seconds to get that back if you never stop shooting. so in order to recover from 1 hit, you have to stand there and shoot long enough for them to hit you 5 more times. or you kite, so lets say it takes about twice as long since you run instead of attack half the time. so 25-30 seconds to get that back. 30 seconds is a long time to never get hit by a mob, when they have things like: mortar, teleport, vortex, wall, fast, devastate, jailer, frozen, arcane, and reflect damage. then some mobs just come with things like: ranged attacks, invisibility, burrow, and leaps.
so option A is loose alot of dps to take 3 hits, or just try to kill them twice as fast perfectly. with a pack in act 1 i can usually burn them down in 20-60 seconds, it makes sense that its better to try a perfect offense and ignore defense completely. my demon hunter friend has so little defense he gets one shotted by the chains on butcher. (15k hp type glass cannon). i go for just enough to survive one hit, and hope i have a potion ready.
so if melee got hit for half as much, and could stand there and dps twice as often, then they can get 4x the survivability from life on hit.