It’s not Newtonian physics, we know Newtonian physics is wrong and have done for about a century.

It’s Einstein physics and quantum mechanics that need to be unified. Of course the fact that they don’t agree probably indicates that one, or the other, or both is also wrong.

I suspect that the real answer probably has something to do with our misunderstanding in how coarse graining plays inro quantum mechanics, because it just seems sort of a cop out to say that changes smooth out over larger distances. It’s just putting an arbitrary boundary between the two realms. It’s a good explanation but it also doesn’t really make any sense because why would the universe care?