Sargeras has always been able to sense World Souls. Even Aggramar sensed the nightmarish dreams of Azeroth when he first found the planet. We know now that Argus probably has a World Soul. Why didn't he sense it when he arrived to recruit the Eredar?
And for that matter, when did Sargeras intend on destroying Argus? His quest is to eradicate all life. Since he couldn't exactly destroy the planet of those he promised great power to, did he know Argus had a World Soul and planned to go back shatter it when KJ and Archimonde were destroyed?
Is he no longer able to destroy planets because his sword was split in half?