The Outcast Dead ++[SPOILERS WITHIN]++

[Nefarius Drapesh]

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...

[Psychosplodge]

Well think back to the earlier books in the heresy and how civilised some of the legions were depicted as. There would be cause to keep a few around, maybe at a reduced size, to keep over ambitious governors in check. And of course you'd need the Wolves around to keep them in check.

[Katharon]

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...

I will refer you to the quote from First Captain Sigismund of the Imperial Fists from Horus Rising: "We will spend our lives fighting to secure this Imperium, and then I fear we will spend the rest of our days fighting to keep it intact... In the far future, there will be only war." Even if the Crusade had been allowed to be complete and the full extent of the old human colonies re-connected with Terra, there would still be a need for the Astartes. Even if, as the Emperor was aiming for, Chaos was dealt a fatal blow by the elimination of all human-spawned religions and faith in Chaos; you would still have the Orks, Eldar, Tau, Necrons, Tyranids, and myriad other foes to still deal with.

[Bob821]

RE-read Outcast Dead then. They spell it out quite clearly in the book that it was intentional.

More than once if my memory serves.

[miteyheroes]

During the Siege of Terra, the Emperor's Children leave the siege and start experimenting on the civilian population. I'd be absolutely astounded if Fabius Bile and the Thunder Warriors don't end up meeting...