I certainly wouldn't want any armies to wholly disappear. They all add something to the lore, and all have fans with full collections of said army.
As for merging armies, perhaps the Marines could be folded together. Though how smoothly that would go in practice and how much the
DA,
BA and
SW could lose from being folded could be problematic.
Also, inb4 "merge the
SoB,
GK and Inquisition".