I was on the anti-ban side at the start of the suspect test and now I'm even more so.
I completely reject the 50/50 argument for a simple reason: the outcome of King's Shield mindgames is in the hands of both players. It's not RNG-based. It's not entirely in Aegislash's favor.
Just because it's not RNG generated doesn't mean it isn't random-- besides, randomness is not the problem.
The problem is that any OTHER Pokemon in the situation where Aegislash uses King's Shield would have NO 50% chance-- but would be simply left with the choice of "sac myself or give the opponent a free turn."
In Aegislash's case, he's given the choice of:
"Sac myself while almost preventing any potential set up because my coverage and power is so broke."
or
"Take the chance to King's shield that MAY give them the chance to set up in some very rare circumstances, but will generally just give me a 50% chance to fuck them over and troll-bitch-slap an enemy that would otherwise solidly beat me."
and given the chance of:
"Maybe he'll brain-fart and think I'll King's Shield, using something useless or switching and giving me another free shot to smack him with my stupidly good Shadow Ball!"
King's Shield gives Aegislash a chance to bullshit it's way out of situations no other Poke would have ANY hope in. It's amazing power and coverage allows it to run King's Shield without a great loss, and the move has a lot of creative utility for a multitude of sets beyond just fucking over physical sweepers that would otherwise destroy it.
Thinking about this as 50-50's is the wrong mindset. Thinking of this as similar to the banned DPP/BW Sand Veil Chomp is the best way-- where an already good Pokemon is given a chance to BS it's way through situations it should rightfully lose.
Most of the attacks you'd use against Aegislash aren't even affected by KS: Earthquake/Earth Power, Fire Blast, Shadow Ball for SE damage, as well as Scald/Surf/Hydro Pump and Thunderbolt for neutral damage. Knock Off and Sucker Punch are pretty much the only contact moves you'd want to use against it, but discouraging the use of such powerful moves is actually healthy for the metagame. And even then, the main user of these moves, Bisharp, actually benefits from KS.
Except that it totally screws over Pursuit, which is otherwise the biggest risk to the Ghost type. Not to mention Crunch. Not to mention Flare Blitz. Or Waterfall.
You can't just shrug off moves like Knock Off not being able to hit it either, since Aegislash being a Ghost (and a Steel-type on top of it!) the options any given Pokemon will have to hit it will be decidedly limited. I mean, just look at how much chaos something as frail as Gengar can cause with Sub-Disable. With Aegislash's sturdiness and resistances, King's Shield can really throw a wrench in a given Pokemon's ability to do anything to it.
Finally anything with stats moves such as Leech Seed and WoW turns this 50/50 into a 0/100: if Aegi doesn't switch out it's getting either crippled or OHKO'd. Forcing something out is considered a successful attempt at checking or countering and since many viable pokemon can do this to Aegislash it's by definition not broken.
Except... Aegislash is the Pokemon that decides when to King's Shield and when to just get out of there. When was the last time you saw Aegislash use King's Shield against something that frequently uses W-o-W or Leech Seed? Not to mention neither of those moves are necessarily a big issue-- I'll often purposefully stay in on Rotom-W's WoW because a burn to my modest Aegi is a small price to pay for a chance of smashing Rotom's often squishier special side with a massive Shadow Ball.
Basically, all your arguments are absurd.