I know from personal experience that Mandi DOES NOT beat Bisharp. Also, even if you kill it, its job (preventing you from Defogging) is done and it has crippled your Defogger to the point where it'll be in KO range of one of its other teammates. That's the beauty of Bisharp; it's either your win condition or opens up an opportunity for another win condition.Actually your calcs show perfectly that Bisharp CANT switch into most Defogers. It takes 50% dmg from Zapdos Thunderbolt when switching in and cant do anything back without +2 which it wont get in that situation. The first knock off does 50% back, but Zapdos can just roost it of or simply kill Bisharp, its also important to note that Zapdos usually outspeeds so Bisharp will have to go for Suckerpunch instead of Knock off.
Against Mandi it can do even less. Bisharps Ironhead does like 30% unboosted, Mandis Foulplay does 30% as well. Difference here is that Mandi has Roost. If Bisharp uses SD it will take ~70% from Mandi and die.
Full def Skarmory can simply defog and phaze bisharp out afterwards. Yes its massively crippled afterwards but its job is done. The only Defoger Bisharp is good against ist Latios/Latias and as your calc shows, he takes massive damage from HP fire to a point where he might even die with SR on the field.
When it comes to killing Defogers and abusing Defiant Thundurus does that job better. He fares far better against Zapdos, Mandibuzz and Skarmory and doesnt have to rely on unreliable priority to sweep.
Also, Thundurus lacks the resistances to switch into Defoggers safely (it doesn't resist ANY of the most common Defoggers' attacks), making it more difficult at doing its job.