So let's use 16k raw ele dmg as a reasonable top-end. Using the 65% version of the keystone that you propose, it would take about 12k armour to match ele resist against that. Throwing 4 endurance charges on top would be the equivalent of (1-[2k+2800*.54]/4k)=12% less elemental damage taken. Upping the armour to 20k and using 5 endurance charges, the keystone would effectively provide (1-[2k+2.8k*.39]/4k)=23% less ele dmg taken.
Yes, you could get crit and the keystone wouldn't provide as much benefit. But you could also stack, say, 40k armour and 7 endurance charges as a juggernaut. At that point, assuming 6k life, you're not getting oneshot by raw elemental hits of less than roughly 34k raw elemental damage (8.5k against max resists). Against the massive 16k raw ele from before, you take 2.3k, a bit more than half what a max resist character would take.
Again, maybe that's fair. Maybe you should be able to be that tanky against most of the game, if you invest in 40k armour and 7 EC's. But it still seems one-dimensional. If you meet the threshold of resists against the largest hit you expect to take, then you take the keystone. If you don't meet the threshold, you don't take it.
In that way it's not dissimilar to MoM (have enough mana? take MoM) but imo the accessibility of max mana, combined with the limitations of mana recovery, make that keystone more interesting to build around while also being open to more different builds and ascendancies (though Trickster and Inquisitor are favored). But MoM shows that Steel Deflection isn't a totally absurd idea for a keystone, I'll give you that.
To me it seems too specific of a certain build type for a keystone. I'd rather see a mechanic like SD on, say, a lowish-armour body armour or shield. Then you build around the weakness of that item. The ceiling of how high your armour can go is limited by this mediocre-armour item that you are forced to run.