When you can avoid the 2hko from moves like landorus-t life orb quake, keldeo secret sword (which is super effective), scarf salamence outrage, band garchomp outrage, +1 rain boosted waterfall from gyarados etc. with just sheer bulk you know youre fine. Porygon2 doesnt need resistances to take on top threats in the metagame, he would definitely be better if it did but it still does very well without them which is the main reason he shouldnt be any higher than B-Rank. Also who cares if its attacks are weak against neutral targets? This thing job is tank hits, status you (toxic and discharge) and restore health. His boltbeam attacks are only used against the stuff that are hit super effectivelly and it hits pretty hard. While i do agree that trick room and gravity are quite gimmick that does not mean they are not effective. With threats like sheer force conkeldurr and reuniclus to abuse trick room and landorus (both forms), nidoking, starmie and essentially anything that learns a bunch of powerful innacurate moves to abuse gravity they can hold their own in this metagame.
252+ Atk Life Orb Landorus-T Earthquake vs. 252 HP / 184+ Def Eviolite Porygon2: 140-165 (37.43 - 44.11%) -- 1.56% chance to 2HKO after Stealth Rock
252 SpA Expert Belt Keldeo Secret Sword vs. 252 HP / 184+ Def Eviolite Porygon2: 182-218 (48.66 - 58.28%) -- guaranteed 2HKO after Stealth Rock
+1 252 Atk Salamence Outrage vs. 252 HP / 184+ Def Eviolite Porygon2: 166-196 (44.38 - 52.4%) -- guaranteed 2HKO after Stealth Rock (I used +1 because more often than not Salamence is going to be at +1 or higher with a pivot switching in, you're arguing he's a pivot anyways)
+1 252+ Atk Life Orb Gyarados Waterfall vs. 252 HP / 184+ Def Eviolite Porygon2 in rain: 224-265 (59.89 - 70.85%) -- guaranteed 2HKO after Stealth Rock
There's all your claims, and the only relevant one I'll give you is the Landy-T one and maybe the Salamence one. All the sets are standard, so don't argue it. I figure I should throw in some more relevant threats also:
252+ Atk Choice Band Scizor Superpower vs. 252 HP / 184+ Def Eviolite Porygon2: 236-278 (63.1 - 74.33%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Choice Band Tyranitar Superpower vs. 252 HP / 184+ Def Eviolite Porygon2: 240-284 (64.17 - 75.93%) -- guaranteed 2HKO after Stealth Rock
252 SpA Life Orb Sheer Force Landorus Focus Blast vs. 252 HP / 72 SpD Eviolite Porygon2: 263-309 (70.32 - 82.62%) -- guaranteed 2HKO after Stealth Rock
252 Atk Choice Band Terrakion Close Combat vs. 252 HP / 184+ Def Eviolite Porygon2: 320-378 (85.56 - 101.06%) -- 81.25% chance to OHKO after Stealth Rock
252 SpA Life Orb Thundurus-T Focus Blast vs. 252 HP / 72 SpD Eviolite Porygon2: 239-283 (63.9 - 75.66%) -- guaranteed 2HKO after Stealth Rock
252 Atk Life Orb Technician Breloom Low Sweep vs. 252 HP / 184+ Def Eviolite Porygon2: 211-250 (56.41 - 66.84%) -- guaranteed 2HKO after Stealth Rock
+2 252 Atk Garchomp Outrage vs. 252 HP / 184+ Def Eviolite Porygon2: 214-253 (57.21 - 67.64%) -- guaranteed 2HKO after Stealth Rock (Yache Berry variants beat you and SD up, non Yache I'll give to you, CB has around 96% chance to 2HKO after SR)
252+ Atk Choice Band Kyurem-B Outrage vs. 252 HP / 184+ Def Eviolite Porygon2: 216-255 (57.75 - 68.18%) -- guaranteed 2HKO after Stealth Rock
+1 252 SpA Life Orb Volcarona Bug Buzz vs. 252 HP / 72 SpD Eviolite Porygon2: 191-226 (51.06 - 60.42%) -- guaranteed 2HKO after Stealth Rock
I think that's enough.
If you notice, almost all of these common offensive threats can either set-up on you or plain out attack you and you're a sitting duck (bad pun). Porygon2 is slower than almost all of OU, which OU loves taking advantage of. So many things can simply just set up on it and then blow straight through your team because you're too busy Toxicing or using Recover because you don't have Leftovers to neutralize passive damage. He's too busy trying to Toxic sweepers that are faster and 2HKO you. Sure, he's good at checking locked-in threats and spreading Toxic, but so are Tentacruel, Jellicent, the Pink Blobs, and Politoed. If he had a better support movepool he'd be better, but passive damage sucks and he's too slow to actually abuse Recover. C-Rank without a doubt.