Sharpedo is worth its ranking in C+. Speed Boost makes it one of the more reliably late game cleaner in the game. Base 120 Atk and being able to afford to run an Adamant nature and Life Orb lets it hit stuff quite hard. Water/Dark is an amazing STAB combination, only resisted by Azumarill, Breloom, Keldeo, Chesnaught, Greninja and MGyara in the whole of OU, and can run Zen Headbutt to further down on the number of resists. Sharpedo has a similar role to Greninja; Offensive teams have a tough time against it. Sharpedo might not be able to take a hit, but due to the nature of how it works, it doesn't have to take any. Destiny Bond is also brilliant for such a frail Pokemon to have.
Aren't you a little concerned that the resists you listed are all mons who can OHKO Sharpedo back?
252+ Atk Huge Power Azumarill Play Rough vs. 0 HP / 0 Def Sharpedo: 728-858 (259 - 305.3%) -- guaranteed OHKO
252 Atk Life Orb Technician Breloom Mach Punch vs. 0 HP / 0 Def Sharpedo: 517-611 (183.9 - 217.4%) -- guaranteed OHKO
252 SpA Keldeo Secret Sword vs. 0 HP / 0 Def Sharpedo: 560-662 (199.2 - 235.5%) -- guaranteed OHKO
4 Atk Chesnaught Wood Hammer vs. 0 HP / 0 Def Sharpedo: 560-660 (199.2 - 234.8%) -- guaranteed OHKO
4 Atk Chesnaught Hammer Arm vs. 0 HP / 0 Def Sharpedo: 464-548 (165.1 - 195%) -- guaranteed OHKO
0- Atk Life Orb Protean Greninja U-turn vs. 0 HP / 0 Def Sharpedo: 343-406 (122 - 144.4%) -- guaranteed OHKO
I'm not even gonna bother posting calcs for MGyara, it's even uglier. These aren't even S tier pokemon who wall and OHKO back - these are A and B tier pokemon, and some of them are pivots (Greninja) or Tanks (Chesnaught) and they still OHKO back. For Sharpedo to work you need to have to many conditions:
1. All priority users are dead/close to dying
2. Opponent hasn't laid down hazards
3. Opposing mon hasn't gotten any boosts.
4. Mons who resist Sharpedo's stabs/Earthquake coverage are gone
5. Physical Walls are dead
That's way too many conditions for a mon that can't even guarantee you the win. Sharpe also gets screwed over by common walls:
4 SpA Clefable Moonblast vs. 0 HP / 4 SpD Sharpedo: 396-468 (140.9 - 166.5%) -- guaranteed OHKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 0 Def Clefable: 230-270 (58.3 - 68.5%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Life Orb Sharpedo Crunch vs. 252 HP / 0 Def Clefable: 114-135 (28.9 - 34.2%) -- 99.7% chance to 4HKO after Leftovers recovery
4 Atk Hippowdon Earthquake vs. 0 HP / 0 Def Sharpedo: 243-286 (86.4 - 101.7%) -- 12.5% chance to OHKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 252+ Def Hippowdon: 226-268 (53.8 - 63.8%) -- guaranteed 2HKO after Leftovers recovery
0 Atk Skarmory Brave Bird vs. 0 HP / 0 Def Sharpedo: 219-258 (77.9 - 91.8%) -- guaranteed 2HKO
252+ Atk Life Orb Sharpedo Waterfall vs. 252 HP / 232+ Def Skarmory: 101-121 (30.2 - 36.2%) -- guaranteed 4HKO after Leftovers recovery
252+ Atk Life Orb Sharpedo Crunch vs. 252 HP / 232+ Def Skarmory: 101-121 (30.2 - 36.2%) -- guaranteed 4HKO after Leftovers recovery
Hippowdon, Clefable and Skarmory are more likely to KO Sharpedo than be OHKO'd back. That is unacceptable for an offensive mon.
As for M-Absol, as frail as it is, if it is somehow allowed to get up a SD, +2 Sucker Punch makes it hard to revenge kill. 115 SAtk makes it hard to fully wall. While its usefulness has fallen, it still has a usable role in the metagame. B- is the tier where something can sweep if unprepared for; C+ is the tier where something cannot reliably sweep even when unprepared for; so B- is ok.
M-Absol is severely pressured to find that SD opportunity though. It's in a terrible position when it comes out - does it Sucker Punch and risk not killing the opponent, and then get killed by the counter attack, or does it Swords Dance only to risk getting OHKO'd? It also can't switch into status moves until it's M-evolved. It can't reliably sweep even when unprepared for because most teams already have powerful attacks/priority that can deal with M-Absol.