Aegislash is not moving out of S anytime soon. Well, I see Aegislash being out of the S rank... because I see going to U rank, alongside things like Mega Lucario and Genesect.
Why? Because I think that...
-Has a myriad of sets, all of them with different checks or very different counters (some of them doesn't have that thing).
-Can use atttacks from both sides oft he spectrum very effectively. Can use mixed sets with little drawbacks, compared to other mixed attackers, who has to sacrifice power in one end of the spectrum AND lose one of the defenses.
-It love being slower than the rest of the team. Many Pokemon (including many walls/tanks) wants to outspeed key threats and they have Speed EVs. On the other hand, Aegislash loves being slower, because it can tank almost any attack (even SE ones) and because his ability, thing that only Gyro Ball users (in OU, mainly Ferrothorn) and Trick Room teams can said about that.
Resist Staleth Rocks. We know how important is this move; compared to other S rank, who are neutral (Landorus) or weak 8thundurus, MCharizard-X) they should not be worried if some rocks are on the field.
-King's Shield. Not only changes forms, but if during the use of this move, a contact move hits it, his attack will be halved. With includes (I consider the relevant ones to a competitive team): All physical Fire Attacks in OU, All Physical Water attacks, All the physical Electric attacks, Many of the Grass Attacks, Ice Punch, Ice Shard, Almost any Fighting attack, Poison Jab, All the Physical Flying asttacks, all of physcal Psychic attacks, almost all of the physical Buig attacks, Head Smash, all the Ghost physical attacks, all the Dragon physical attacks, almost the Dark physiucal attacks, almost all the Steel attacks and Play Rough. This is a lot of attacks, and any physical user at -2. This causes a centralization around Bisharp, Tyranitar, Sucker Punch users and EQ users.
I think that compared to other users, they are better.
-Mega Charizard X: almost always has Dragon Dance, we know it will have physical STABs, special sets are rare and are generally used at lure certain opponents (in particular and for surprise, Ferrothorn). Of course, it's a great S rank pokemon. Also, has two weakness (that unlike it, can defeat it easily), has an insane weakness to SR.
-Thundurus: First, it's rather frail. Second, if the opponent doesn't Defog, a mixed or physical set is considered a bit inferior. Third, Prankster is the superior ability in general, and the one it fits in almost any type of team. Fourth, Has a weakness to SR.
-Landorus. One of the pokemon helped by Aegislash, because Aegislash destroys the set of "Levitating/Flying Psychic types" that walls Landorus to no end (specially Cresselia). Has a very predictable sert (we know (unless some rare Sand Force set) 2 of the moves of Landorus and you can guess the third one). And it's rather frail, as has two common weakness, Water and Ice (the latter is a 4x weakness), which great users like Mamoswine, Weavile, Greninja, Keldeo, Azumarill, Kingdra, etc.
About non-Mega Charizard, E rank at best. In other words, please use a Mega. (The X is the better, but the Y is very good).