Although its true that zapdos is more common, but on the other hand rotom is much less common, defensive lando dont carry stone edge and skarmory is a much better counter to pinsir than celesteela, because it can use counter on pinsir but celesteela cant ohko it.I second this. It was A- last gen and Gen 7 has been very mean to it. Zapdos is now a more common and useful Pokemon that has all the tools to eat Mega Pinsir for breakfast, lunch and dinner. And sure Skarmory isn't as good as it was before but now it has the even better Celesteela to deal with. It also doesn't help that hazard control is a lot worse this Gen, especially for offense. Oh and there's also Magearna and Tapu Koko. I like Mega Pinsir but it doesn't belong in A- right now IMO.
Also hazard control being less common means that sticky webs teams are much better, and pinsir beats literaly all the pokemon you listed there bar zapdos under webs.
Also zapdos cant count it 100% of the time if it swithces in when pinsir uses SD
0 SpA Zapdos Discharge vs. 0 HP / 4 SpD Pinsir-Mega: 228-270 (84.1 - 99.6%) -- guaranteed 2HKO
+2 252 Atk Aerilate Pinsir-Mega Return vs. 248 HP / 244+ Def Zapdos: 183-216 (47.7 - 56.3%) -- 30.5% chance to 2HKO after Leftovers recovery
With stealth rock is a clean 2hko