B- is very appropriate for Weavile if you ask me. Knock Off is a huge buff and great new STAB to abuse. Seriously, Dark and Ice STABs are dastardly good to have, and it's virtually unrivaled speed tier gives it some of the fastest priority available. And with the combination of STAB Pursuit and Ice Shard it's effectively one of the best naturally fast revenge killers available without needing to use a scarf. Obviously its ridiculously bad defensive typing and frailty make him a bit hard to use. He's high risk high reward where your margin of error with him is small, but in expert hands he can be a deadly assassin. Here are some of the top threats he manhandles:
Thundurus
Landorus
Lati@s
Zapdos
Mega Pinsir
Aegislash
Gliscor
Bisharp with Low Kick
Talonflame with Stealth Rock
Tyranitar with Low Kick
Gengar
Competently B-, not hard justifying a team spot.
Just to point this out, Weavile can't counter (or even win one on one) a lot of the pokemon that you listed.
-1 252 Atk Life Orb Weavile Knock Off (97.5 BP) vs. 252 HP / 0 Def Aegislash-Shield: 182-218 (56.1 - 67.2%) -- guaranteed 2HKO
-1 252 Atk Life Orb Weavile Knock Off (97.5 BP) vs. 0 HP / 0 Def Aegislash-Shield: 182-218 (69.7 - 83.5%) -- guaranteed 2HKO
252 Atk Aegislash-Blade Sacred Sword vs. 4 HP / 0 Def Weavile: 620-732 (219.8 - 259.5%) -- guaranteed OHKO
252 Atk Life Orb Weavile Ice Shard vs. 4 HP / 0 Def Mega Pinsir: 140-166 (51.4 - 61%) -- guaranteed 2HKO
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 4 HP / 0 Def Weavile: 276-325 (97.8 - 115.2%) -- 87.5% chance to OHKO
252 Atk Life Orb Weavile Ice Shard vs. 4 HP / 0 Def Talonflame: 107-126 (35.9 - 42.2%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Choice Band Talonflame Brave Bird vs. 4 HP / 0 Def Weavile: 334-394 (118.4 - 139.7%) -- guaranteed OHKO
Sure none of these pokemon can counter Weavile themselves, but they certainly aren't bait for Weavile.
The prevalence of strong priority in OU, in conjuncture with its terrible bulk and stealth rock weakness make it very hard to keep alive throughout the match. Additionally, the relative decrease of threatening set up sweepers with 4* ice weakness (and there's always the terrifying prospect of weakness policy Dragonite) has made Weavile's niche even smaller in OU. It still has a few strong points namely its crazy speed in a meta with few scarfers and a buffed knock off, which I think is a decent enough reason to put it in
C+.
Edit: I also disagree with you on saying that its easy to justify giving Weavile a teamspot. I think people often understate the importance of defensive synergy, especially on HO teams. Even HO teams, where Weavile is often found, rely on defensive synergy to switch into resited attacks and keep the momentum. Weavile provides almost no defensive synergy whatsoever, and that really shouldn't be overlooked. (Deoxys-S and other suicide leads are different as they have an extremely specific function for the team and are only meant to be used once.)