James already has other blood heirs... remember Jordan Sinclair - alternate son of James and the real father of baby Cabot - thus James has both another son and a grandson already.
Secondly, Dusty (albeit the real heir to the Stenbeck fortune/title) is not recognized by James as being an heir and is also not the biological father of Johnny. Thus James would have no concern about them - other than to kill Dusty off to keep him from claiming title to the Stenbeck fortune... which he never has tried nor likely will try to do.
James wanted Cabot - not Johnny. And I would love to have Jordan and Cabot back too!