User blog comment:Supahbadmarine/By Tzeentch! Here We Go!/@comment-9465750-20130212130355/@comment-3258762-20130212191231

You are underestimating the hatred the Loyalists hold for the Traitors and vice-versa. They would much rather side with the new Primarchs than join those who committed the Drop Site Masacre and the Siege of Terra.

Note: Them joining the New Primarchs would not a big chance at all. Probably, it would just end with the very untrusting old Primarchs vs the new and chaos ones.