I interpreted the role of the Thunder Warriors in Outcast Dead more as a metaphorical analogism to the Space Marines.
First pinnacle of bioengineering and warfare, but when the mission/conquest is done they become useless. More so, they become an unwelcome leftover.
If the Horus Heresy didn't happen and the Space Marines stayed unified and loyal, as soon as the galaxy was conquered that would've also been a possible fate for them.
Without war no use for warriors...