Reading "Mao: the unauthorised biography" - there were SO MANY opportunities for Chiang to wipe them out both before and during the Long March, when they were strung out, starving, and with no ammunition but what they carried from the start, which was not much. Chiang intervened personally and repeatedly, removing commanders who pursued them too aggressively.
Chiang planned to take care of them eventually I don't doubt, but his priorities was destroying autonomous provincial governments and amassing a private fortune.
Chiang's only son, interestingly, was an avowed Communist and spent the war in Moscow.