Mitigation and avoidance has exponential returns per point for their relative damage reduction. If you go from 99% to 100% mitigation or avoidance, you become invincible.
The problem is mitigation takes exponential amounts of armor to increase per point.
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Which is exactly why armor is a linear stat, just like stamina, strength, attack power, spell dmg... Of course for some classes in certain cases the increase isn't completely linear, but for most stats in the game for most cases it's pretty close to linear. Armor is no different, as the amount of unmitigated damage you need to take before you are killed increases linearly with armor.
Of course avoidance has exponential returns when it comes to "with X points of healing available, how long would it take me to die?" however it does nothing when the worst case scenario comes up, and it will come up sooner or later. The only way I could see avoidance being a good thing is if a burst that can kill you is so rare that it will happen on a low % of the full fights (so, say, you define it as "OK" to get gibbed once every 10, 100 or whatever fights), and the biggest burst that is likely enough to happen is too small to be a concern.
The reason for every linear stat having its relative increase with diminishing returns is easy to explain without math really - when you have 50k HP 5 more HP isn't as noticeable of a benefit as if you had 10 HP. Relative increase is increase/current, which for a fixed increase goes down as your current goes up.
Another way to look at it is that armor multiplies your HP in terms of "how much unmitigated damage you need to take to die", so same amount of armor is worth more with more stamina. This works the other way around, stamina being better when you have more armor.
As for BV, I it's linear for threat, not survivability. For survivability it's exponential, yes, but not as much as you think. In fact with most bosses the survivability gain from block value is quite close to linear.
Just like saying that 10%->11% avoidance will give 1% less damage taken is not far off at all. Same with BV, since you can almost say that BV<<dmg then you can say the increase to survivability from BV is not very far from being linear. If you tank something that hits for something closer to how much you can block then yeah it's going to be a lot more far off.