I don't see a reason to drop Doublade to A- over A or A+, it has the ability to pull off a few sets and just be a menace to teams that don't bring a bulky water or something that can revenge it easily. I think in order to demonstrate this we will need to go over all the sets it can pull off:
Doublade @ Leftovers 
Ability: No Guard 
Level: 50 
EVs: 244 HP / 12 Def / 252 SpD 
Sassy Nature 
IVs: 0 Spe 
- Gyro Ball 
- Shadow Sneak 
- Swords Dance 
- Toxic
Probably my favorite Doublade set. It allows Doublade to tank almost any non-boosted hit even SE hits and simply be a very annoying wall. Steel-, Ghost-typing is a great defensively allowing it to take on Fairy, Flying, Rock, Normal, Steel, Grass, Bug, Fighting, Poison, and Ice (That's all I can think of off the top of my head). This allows Doublade to switch into plenty of Pokemon and really threaten them out with its superb attack. It's typing allows it to using certain Pokemon as complete setup fodder. Common spikers like Roselia can't hit it, even a lure set with HP fire and 252+ SpAtk can't 2HKO and gets KOd by a combination of Gyro Ball and Sneak. Common defoggers like Crobat also can't beat Doublade and their only hope is to run bulky taunt to stop Doublade from SDing and really hurting something. Toxic allows Doublade to punish things like Porygon2 that may want to switch into a Gyro Ball and P2's best option for hitting Doublade is HP Fire or Thunderbolt. However this set has issues with many of the powerful Physical wall breakers and with Pokemon that can abuse No Guard with WoW. Doublade also can have trouble getting the full power out of Gyro Ball because of how slow the tier is as well as because many fast Pokemon in the tier like Electabuzz resist Steel-type moves.
Doublade @ Life Orb 
Ability: No Guard 
Level: 50 
EVs: 52 HP / 252 Atk / 8 SpD / 196 Spe 
Jolly Nature 
- Autotomize 
- Swords Dance 
- Iron Head 
- Shadow Claw
This set is really great at capitalizing on Doublade's great offenses and nice boosting moves. This set carries dual stab which hits almost everything for neutral damage which is fine because after one SD Doublade hits 324 Attack which is absolutely insane. After one Autotomize you outspeed the whole unboosted metagame. And you have an Atk stat of 162. There isn't a whole lot to say about this set, just rely on your bulk and offensive pressure to force frail things like Roselia out and use that as an opportunity to setup and SD or Autotomize. If you have +2 in both there isn't much that can take a hit and it comes down to things like scarf revenge killers to stop you.
I think that it makes sense to drop Doublade, it is good but I don't see why it is S tier, there are plenty of things that can rip it apart and plenty of things that can tank its hits; at the same time it is hard to for sure say it should drop. Doublade is very much on the fence. It has good typing and stats but it is in a meta that can give it some trouble with Piloswine and Rhydon running around as well as many things being able to quickly wear it down and abuse its awful speed. Its ability hurts it more than it helps it; Doublade doesn't rely on shaky moves yet Pokemon do rely on shaky moves to beat it (WoW).
For my final consensus drop it to A+/A, A- is taking it too far.