I'd also mention that the indomitus box checked off multiple things on the community wishlist(make new xenos, baroque primaris, melee primaris) and even some faction specific ones (necrons have wanted the cryptek variations and updates to 3rd models for a while) while dominion didn't really hit any of the big ones (k boys aren't really a new destruction faction, I think people more accept d the new stormcast rather than actively asked for the change).
Gw shouldn't have expected this to sell half as well as indomitus especially since warclans and stormcast I think combined are about half.the playerbase percentage wise of space Marines.
On the bright side, if this is a flop for gw, it's good news for skaven and craftworld eldar players