Board Thread:Warhammer 40k General Discussion/@comment-4851119-20140118095944/@comment-3258762-20140121151002

The Thunder Warriors had one purpose: To conquer Terra. The Astartes have one purpose: To conquer the galaxy. First it was intended that the Primarchs would lead normal mortals into battle, but when they scattered he made the best of the situation.

The Thunder Warriors were never outdated, they were rendered obsolete. Their lifespans were too short to allow for long extraterrestrial campaigns, so the Emperor had them put down.

When no enemies remained in the galaxy, would the Emperor really have kept the Astartes?

Further, Astartes were never the pinnacle, they were actually inferior in almost all combat regards to Thunder Warriors. It probably had to do with the very dodgy methods of creating a Thunder Warrior that might have allowed them to be superior to an Astartes but wouldn't be any guaranteed result.