That's because you don't actually take 4.9 hits (well you might take .9 of a hit, that gets into my whole stochastic argument and that's kinda different from this, but I'll show my point in a second). A boss hits an integer number of times. So if you can survive anywhere from 4.0+ to 5.0- hits, this means that the boss will actually swing 5 times and the 5th one would kill you, therefore the model is relevant in terms of the beneficial impact from avoidance.
5 is better than 4.9, but in the model we're considering it the same.
And ya,the black heart is totally better than Eitrigg's Oath =P.
The whole overarching premise though, is that as time -> infinity, you're gonna take that hit string that RNG can't save you from. Can a marginal increase in stam give you that same safety? Not technically, but it is the only thing that is guaranteed to have some marginal benefit. Those sums added can add up to a lot, more than just the individual parts, and those are the only things that give healers a buffer to insure that they increase HPS > DTPS or for a tank to use a cooldown. That's the whole point. That's why tanks live throughout fights, because even if they take an unlucky hit string like that, the healers have a buffer to heal the tank back up to full.
"If the world is something you accept rather than interpret, then you're susceptible to the influence of charismatic idiots." -Neil deGrasee Tyson
Twitter @Aggathon || @Tankspot || Twitch.Tv/Aggathon