I don't know about Cha Bum-kun because he played in a different era, but Son Heung-min isn't S-class

He isn't the type of player who suits positional play.
Nowadays, all big clubs base their tactics on positional play (since the Pep revolution),
but he has almost nothing he can do in that setup.
People say if he goes to Man City, he'll score a lot by receiving passes from De Bruyne,
but that's only if he's used as a striker, and his ability as a striker is lacking.
As a winger, his scoring ability is good, but Pep never uses wingers as primary scoring resources.
What's truly amazing about Cha Bum-kun is
that he grew up in Korea during the 80s when football knowledge was primitive, served 3 years in the military, and still dominated in Europe.
However, even so, it's still an unknown how well he would perform in the current era.