Same can be said for Reuni then, with one setup it can be heavily abused.
And that is why Serp wont get banned.
"High level players" have a tendency to abuse a lot of things, that is a whole different arguement and more skill based rather than an unbalanced meta.
The highest win rate of a mon in ou is Shedninja, which is ironically the least used mon too but that really proves the use of intellect and skill rather than a broken ability. Empoleon and Weezing have higher win rates too and Golbat still has one of the highest win rates for months now and it is used as a Serp check so it is getting the job done right? Also Serp isnt in the top 10 most used mons, hence people arent running around spamming Serp.
A part of every team is built around the fact that it needs to counter Rain in OU, that is the effect rain has my statement is simple - if people found ways around rain then I do not think Serp is op than rain.
252+ SpA Abomasnow Blizzard vs. 56 HP / 0 SpD Serperior: 170-204 (108.2 - 129.9%) -- guaranteed OHKO
252 SpA Life Orb Dragonite Fire Blast vs. 56 HP / 0 SpD Serperior: 143-169 (91 - 107.6%) -- 43.8% chance to OHKO
252+ SpA Life Orb Dragonite Draco Meteor vs. 56 HP / 0 SpD Serperior: 140-165 (89.1 - 105%) -- 31.3% chance to OHKO
+ Pairing up with priority moves like BP, ES, IS and Mach + Hail/Sand + Hazard damage is more thane enough to get the job done without even using Chansey. I cannot introduce a whole gameplay to prove you a point, but honestly Sepr isnt broken at all.