Khan (and Joachim) never seemed interested in Earth at all. Probably they would seek a planet to conquer. I remember a passage in the book when Khan examines his recently aquired Genesis torpedo and thinks something like "when he tired of ruling worlds that existed he would make new world to rule."
And of course, without Kirk, when the whale probe came Earth would be fubared. Kruge and Valkris would never intecerpt any transmission never know about Genesis and die of old age. Or not, because without Earth, when Praxis exploded the Klingons would be fubared. Without Earth there would be no Enterprise-D, then Soran would get in the Nexus and probably the Viridians would be fubared. Voyager would never exist so the Borg wouldn't be fubared, so the entire delta quadrant would. Until species 8472 came to fubar everything.