I like how you seem to suggest that a 'filler' raid is like throwing together a couple of world quests on a slow afternoon. "Got that raid you asked for, boss."
What about art assets for the raid? Lore to connect it to the Legion story? Physical location on the map? Art assets for the loot? What would the loot be used for, given the final expansion boss and the tier races would be done? Even 'filler' raids require a buttload of planning, design, and development. Each hypothetical '7.4' people theorize to come at the end of Legion is essentially an 8.1.5 not being implemented in the next expansion.