You guys are missing the point. You are seeing faction building as an optimization process from the point of view of players with big collections. AoS was born to sell more kits, because WHFB was so consolidated that the cycle of selling kits was reduced to selling new army book and couple of kits to people with large collection focused on one or another army.
AoS (and new 40k) are meant to stop this from happening: have and create so much material that you are not trapped into the new edition, new army book, couple kits cycle. The idea is to nudge people not to only have a couple of armies and wait for next edition update, but keep experimenting with new factions, alliances, etc.
Small factions fulfil two roles: keep the illusion that almost all old models have a place and a future in AoS, and keep some room for low cost clever experimentation (FEC, BCR, Bonesplitterz) or salvaging into a really new faction (Sylvaneth, DoK).
This is all quite a genius move from the Kirby administration, that was however so cynically and badly managed that blew on their hand. Roundtree’s Golden era for both GW and customers is simply adding good execution and clever price management, plus better consideration of the customer.