I said it before and I said it again: Starmie should stay C+
Aegislash wasn't its main problem. It could theoretically 2HKO it with Analytic HPump. However, here are Starmie's real problems which prevent it in my eyes from moving up:
1. There are better offensive spinners, such as Excadrill and Mega Blastoise, who have higher firepower, can take hits easier thanks to their excellent typing / bulk, and can actually, you know, SWITCH IN on the spinblockers to remove them. On top of that, Mega Blastoise has better coverage and isn't Pursuit/Sucker Punch bait. (also, shut up about Starmie's speed, why not use Greninja at that point + a Defog user/one of these spinners, which you likely need anyway?)
2. The Defog buff really hurt Starmie. Latios, with his much better special bulk and access to Defog STAB Draco Meteor, is one of the main reasons Bisharp became so good in this generation. While Latios can attempt to play mindgames with it, Starmie gets buttfucked if Bisharp switches in, since Rapid Spin is a damaging move and Bisharp has almost no reason not to Sucker Punch. Also, Zapdos, Mandibuzz and Skarmory, with their much higher bulk and access to Roost, are much easier to switch in than Starmie, not to mention these things counter many dangerous threats: Zapdos can check Landorus, Thundurus and Mega Scizor, Mandibuzz wipes the floor with Bisharp and Breloom, while Skarmory can wall Mamoswine, Breloom, Bisharp, Tyranitar, Mega Mawile and Mega Pinsir.
3. Greninja is another thing you should keep in mind when considering Starmie. Greninja is faster and stronger than Starmie, since it gets STAB on all of its attacks and has (slightly) higher Special Attack. The only perks Starmie has over Greninja are Rapid Spin, Thunderbolt and Recover.
4. Starmie has 4MSS. It can't run Hydro Pump / Ice Beam / Thunderbolt / Psyshock / Rapid Spin / Recover at the same time. Without Thunderbolt, it has no hopes to break through (Mega) Gyarados and Azumarill. No Ice Beam means your Starmie wouldn't be able to deal with Thundurus, Landorus, Dragonite and Lati@s. No Psyshock? Mega Venusaur stomps you. Recover would be nice to use since it could heal up while the opponent's spinblocker or wall switches out or uses a status move, which Starmie can cure by simply switching out if it has Natural Cure. No Rapid Spin? Why not use Greninja?
5. Starmie is Pursuit bait, has crappy bulk, can't switch in on anything and its base 100 Special Attack isn't that great anymore with this power creep.