You're correct. In a lab setting, 0C and 100C are not arbitrary.
In the weather forecast, they are.
Which ties into your final point, it's hard to define a scale that is best for everything, which is exactly what I've been saying this whole time. Fahrenheit is better for some things, Celsius for others.
The only reason people in this thread are saying otherwise is because for some reason they've tied up some significant part of their self-worth into their belief that "lmao DAE fahrenheit bad amirite??1?", and they mistakenly believe that those of us that understand nuance are trying to belittle or disparage them in some way. I assure you, we are not.
"Americans" ah, I see. You don't actually care about effective systems of measurement, you just want to shit on people that are different from you.
Also, as answered in another post: Why would you assume that humans, an endothermic species, prefers exactly 50% thermal energy? Of course we sit around the 70F region, we're warm-blooded mammals. We don't want to be half cold, we want to be mostly warm.
No matter how much you complain or argue, it's never going to be true that Celsius is the one-and-only most perfect system of temperature measurement. The fact is that both systems have their applications, as any intelligent member of the scientific community would tell you.
Get over it.