Earlier quoted context omitted.
McWhorter is a linguist (as well as a political commentator). His argument isn't that English is easier because people already speak it natively. Instead, English lost a lot of its complexity (especially around suffixes) during the Viking invasions when it was taken up by adult speakers. This doesn't mean that learning the language is easy--no language really is. And English has some things that make it harder to mas…
English's large vocabulary actually makes it easier as a second language. Adults are really good at learning new words, but struggle with morphological and gender systems. Complex morphology seems to have some benefits that push languages towards including them, but are only sustainable when those languages are predominantly learned by children, who can pick up morphological complexity easily, not adults, who can't.
You have no idea :)