It's not true.
"King Charles" (i.e. the Emperor Charles V) was born in Flanders and spoke no Spanish when he arrived in Spain, and was very unpopular because of that.
But he was very well educated, and a great patron of the arts, so he did make an effort to learn.
His Habsburg underbite might have caused him to lisp, or it might have been an accent. But the Spanish would not have adopted it because of him, whatever the reason.
Proper Spanish may be in the minority worldwide but it's the correct version. After all, the country is called Spain and the language... Spanish. Not Argentinian or Mexican or what have you.
Besides, if you wanted to learn "American Spanish" because there are more speakers of that variety, which one would you choose? Bolivian, Argentinian, Dominican, Cuban, Argentinian, Chilean, Venezuelan, Mexican, Puerto Rican... etc etc? There is no such thing as "American Spanish" as one entity clearly differentiated from "Castilian Spanish", apart from the famous "lisp" (which is no such thing, simply the correct way to pronounce the phoneme /z/
). Chilean speakers mock Mexicans, and Cubans say they don't understand Uruguayans because they "speak funny". You know, not unlike Canadians, Australians, USians, South Africans, Kiwis...