I'm actually a fan of health being the primary defensive stat - mostly because health has severely intrinsic features that resistances don't, that lets it be more easily controlled.
1. Health doesn't reduce the effectiveness of lifesteal type effects - or the secondary effects of an opponent's abilities.
2. Raw health isn't multiplicative with other healing or regeneration effects - allowing it to be effective without drastically multiplying in value with other effects (self heals, shields and the like.)
3. Health is a more obvious indicator of high durability than resistances - just due to how basic UIs work.
4. Because of high health per level gains, the relative effectiveness of health is far more temporary than resistances - This is just a natural fallout of our stat per level gains, not an intrinsic advantage of health.
For example, let's take the following case here:
You have a Bruiser with 1000 health, 300 armor fighting an AD carry, versus a Bruiser with 2000 Health and 100 armor.
1. % Armor Penetration is more effective in the first case than the second as % armor penetration values go up - values start to skew drastically as % armor penetration goes beyond ~20% or so, relatively neutral for low values of % armor penetration.
2. However, Life Steal - is far more effective in the second case - whereas it is completely destroyed in the first case.
3. The Bruiser's life steal / shield statistics are also neutral in the second case - whereas they are multiplied in the first case.
Thus, the AD carry or any damage dealer actually has multiple paths to deal with a high health target than they did - because the other defensive aspects of the bruisers *aren't* being multiplied or their own secondary defensive statistic (lifesteal) isn't being diminished as well.
The flip side of this is that Flat Armor Penetration, the primary statistic of assassins, can be weaker and still be as effective versus Bruisers because of their ability to negate a far larger portion of armor.
Naturally, this has the flipside of making certain resistance focused tanks weaker - but that's something I feel like we should solve on resistance based tanks. So please, if there's a particular resistance based tank that feels weak, let us know.
Let's take another example, in lane:
Let's say I'm fighting a primary physical damage dealer, whose probably itemized some sort of flat penetration. I have something like 800 health and 50 armor. How much gold do I need to spend to double my life-span?
1. Well, I could buy 800 health and have another 1200 effective health! Success!
2. Or I build 150 armor and have another 1200 effective health!
However, let's take a look at how things change when the game continues - as the game continues, the relative contribution of that armor to my health levels doesn't decay mostly because our game awards much more health per level than armor per level. In fact, by the time I'm level 8 or so, I'll probably have 1200 base health - and thus that 800 health is only increasing my effective at a lower ratio.
However, that 150 armor advantage is "still there".
Note this isn't an intrinsic advantage of health versus armor - just a side effect of how we award stats per level - but it does mean that the relative advantage of health in this case is temporary - rather than one that persists through the game.