The thing here is that it is
excessive corrosion.
A bike can't be compared to like for like to a car...it's true that most cars get minor corrosion underneath where they are not visable, and so long as this doesn't become excessive, it's not a problem.
But if the body panels corroded, even slightly, and especially after 18 months!, then you would make a claim under warranty....even though the car is still 'fit for purpose' ,and ultimately the damage is purely cosmetic.
It's basically unacceptable...purely because it shouldn't happen..not for any mechanical infringment.
But isn't that the point???.....body panels shouldn't rust, so you don't accept it, you don't expect a shabby car in 18 months regardless, why should you a bike???...just because it's a bike???
With a bike, especially a naked bike, nearly everything is visable, and also exposed...so therefore it should maintain a level of resistance to the elements, whilst also remaining 'cosmetically' appealing. (A BMW dealer will soon knock off hundreds when talking P/ex deals because it is obviously a big concern with bikes what they cosmetically look like).
Only good build quality and design will allow this.
This is not good build quality, and unacceptable in my view, regardless of conditions it's suffered. If (sorry Wapping!) it was 5/6 years old and had endured a hard life, then you'd expect it....not after 18 months.