Just think of it – if the USA or Canada decided to have Class C over D as per Australia at their 350 Class D airports, the Class C “steps” would extend to 120 nautical miles where they contacted the Class A airspace at FL180. The Class C would cover most of the continent at this level and the “steps” would make the airspace so complex that no pilot would be able to fathom out where the boundaries were!
The whole system would basically grind to a halt.