My assumption is that they looked at their sales numbers and realized they had no way of making SoS popular enough to support so just slapped some titties on Custodes as part of phasing SoS out.
That's possible, but it's also retarded. Not what you proposed, just the possibility of GW doing that. Previously with the HH stuff there were a bunch of SoS units that never got models, and some that only had FW upgrade sprues.
Dual wielding SMGs? Just a FW sprue with arms
Chicks in power armor on jetbikes? Never got models
Squads of women with various cyber animals like huge dogs and fucking alligators to hunt down psykers? Never got models.
Squad of women with long range rifles, never got models
And this is all shit that could have shored up weaknesses within the custodes army in 40k, if they didn't give the custodes themselves jetbikes. Basically all of the infantry fast attack/cav/shooting stuff that the army is or at least was missing all without doubling up on Sisters of Battle(SoS had a flamer unit, that probably would have been too similar). Instead of interesting shit, it was 1 kit with bolters, 2h swords on a t3 body, and flamers(basically just acting like SoB), and 1 character.
But GW being GW continued to pump out space marine lieutenant #27, and a new primaris vanguard vet that's damn near identical to the previous vanguard vets. Gee, I wonder why the SoS products aren't selling... maybe because they don't exist?