Does anyone honestly think Mega Medicham is going to stay in OU for long? And for that matter, Pinsir aswell?
Lemee say my thoughts on this, if you don't mind.
None of those Pokemon you listed are exactly amazing counters to Mega Pinsir. Both Zards, Talonflame, and Garchomp cannot switch-in at all to Pinsir, and TTar only really checks it if its choice scarf or if it is super defensive + has the proper resist berry for its common coverage moves (shuca for eq, chople for cc). Also, Talonflame doesn't exactly run Adamant anymore as it wants as much speed possible so it outprioritize Weavile, outspeed Raikou, etc. CB has also fallen in popularity in favor of SD and SpDef sets. Mega Pinsir can also beat Talonflame if it uses Feint instead of Quick Attack (yes its viable) and Talonflame is weakened enough. It also loses if Talonflame is running a bulkier set w/o bulk up and Pinsir is healthy enough to live an attack. I don't really see CB Garchomp, as it uses Rocky Helmet / Lum / LO / Garchompite more often in my experience. TankChomp also gets crushed when Pinsir sets up an SD. Also, 65 / 120 / 90 bulk is pretty damn solid for an offensive mon such as Mega Pinsir, and 105 speed + priority isn't bad either.
Calcs:
+2 252 Atk Aerilate Mega Pinsir Feint vs. 0 HP / 0 Def Talonflame: 193-228 (64.9 - 76.7%) -- guaranteed 2HKO
0 Atk Talonflame Brave Bird vs. 0 HP / 0 Def Mega Pinsir: 186-222 (68.6 - 81.9%) -- guaranteed 2HKO
252 Atk Aerilate Mega Pinsir Return vs. 248 HP / 0 Def Talonflame: 328-387 (91.3 - 107.7%) -- 43.8% chance to OHKO
The recoil from BB puts it in range of an OHKO.
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 164+ Def Garchomp: 399-469 (95 - 111.6%) -- 68.8% chance to OHKO
As for Mega Medicham, it is an excellent breaker of those fat teams that usually wall everything, and it has double priority to trouble offensive teams as well. Talonflame, Azumarill, Mega Pinsir also can't switch into it at all as well. It does have its problems with its average speed and mediocre bulk, and the Birds do trouble it greatly, but it does have ways around this like Bulk Up, Sub, and priority (obviously can't fit all this into one moveset but proper teammate support will always helps cover its weakness).
252 Atk Pure Power Mega Medicham Zen Headbutt vs. 240 HP / 0 Def Azumarill: 262-310 (65.3 - 77.3%) -- guaranteed 2HKO
252 Atk Pure Power Mega Medicham Ice Punch vs. 0 HP / 0 Def Mega Pinsir: 234-276 (86.3 - 101.8%) -- 12.5% chance to OHKO
252 Atk Pure Power Mega Medicham Zen Headbutt vs. 248 HP / 0 Def Talonflame: 288-340 (80.2 - 94.7%) -- guaranteed 2HKO after Leftovers recovery
Offensive Talonflame gets destroyed. 252 Atk Pure Power Mega Medicham Zen Headbutt vs. 0 HP / 0 Def Talonflame: 288-340 (96.9 - 114.4%) -- 81.3% chance to OHKO
Only time will tell if they stay OU, but the truth is that they are both effective in OU rn and their spots in OU will most likely stay for awhile.