your definition can be applied at 80km. Is there any objective difference when u pass from 99km to 100km? No, there isnt. The reason 100km was used is because its a nice round number, thats purely it. It's not because planes cant fly that high, and the atmosphere extends well past 100km. The point is, its just an agreed upon number by an international group to use as a reference point and for legal reasons. Thats arbitrary. Now if u can define the precise difference between 99km and 100km, then its not arbitrary. Good luck with that tho

Its 100% arbitrary. It's a nice round number. Density is variable and 99.99% of planes r not capable of flying anywhere close to that altitude. And even above that altitude the atmosphere can produce significant drag on space craft.

Not exactly. It's an arbitrary definition of 100km altitude. As the atmosphere doesn't exactly have an end, and outer space doesn't exactly have a start.