There's a simple solution for countering LO Aegis: Put some Def EVs on Metagross (specifically, 96 so that it always survives +2 Aegislash Shadow Sneak). At this stage it once again becomes a counter. And you can always put more Def EVs in if you don't want to rely on a defog user.+2 252+ Atk Life Orb Aegislash-Blade Shadow Sneak vs. 252 HP / 0 Def Metagross: 330-393 (90.6 - 107.9%) -- 43.8% chance to OHKO
+2 252+ Atk Life Orb Aegislash-Blade Shadow Sneak vs. 252 HP / 0 Def Metagross: 330-393 (90.6 - 107.9%) -- 81.3% chance to OHKO after Stealth Rock
+2 252+ Atk Life Orb Aegislash-Blade Shadow Sneak vs. 252 HP / 0 Def Metagross: 330-393 (90.6 - 107.9%) -- guaranteed OHKO after 1 layer of Spikes
(Note Spikes are more common in MonoGhost than SR & even in MonoSteel they are an easy fit)
As you can see Metagross can't switch in. Meanwhile
252+ Atk Metagross Earthquake vs. 252 HP / 0 Def Aegislash-Shield: 174-206 (53.7 - 63.5%)
As you can see Aegis can just tank the Earthquake & set up yet another Swords Dance, no Life Orb needed
+4 252+ Atk Aegislash-Blade Shadow Sneak vs. 252 HP / 0 Def Metagross: 384-452 (105.4 - 124.1%) -- guaranteed OHKO
+2 252+ Atk Life Orb Aegislash-Blade Shadow Sneak vs. 252 HP / 96 Def Metagross: 307-361 (84.3 - 99.1%) -- guaranteed 2HKO
Next. Shadow Sneak has priority. Therefore, if it's gonna attack you, which it probably will at +2 with LO, it'll be in blade forme when you hit it. You've taken some EVs out of Attack to use in Defense, so that leaves you with 160:
160+ Atk Metagross Earthquake vs. 252 HP / 0 Def Aegislash-Blade: 400-472 (123.4 - 145.6%) -- guaranteed OHKO
This means Metagross is a perfectly good Aegislash counter so long as you don't play stupidly and let it set up before switchin Metagross in. Hint: If you see Aegislash, switch Metagross in straight away. You win one-on-one, and suddenly their main threat is down. Hint number 2: If you let it set up to +4, the problem isn't the pokemon, it's the players. In any case, Leftovers Aegislash is more common so you're unlikely to need the extra Def EVs. If you don't believe me, it looks like I'll be playing two people to prove them wrong.
So Aegislash is difficult for psychic, sure, but there's always going to be an element of that in monotype. Every type has something they don't like. It doesn't mean they're automatically defeated by the pokemon though, and Aegislash is far less of a problem than Talonflame for the reasons I've mentioned in the past.