Loot from the last boss doesn't matter because after defeating it you have beaten the game and don't need more gear. I believe there should be more focus on other types of rewards like cosmetics, mounts and titles.
An idea I've always liked is getting an item you infuse your character with to make it more powerful. It doesn't need to be so powerful that it makes it completely mandatory to have killed the boss, but still enough to give an edge going into the next raid tier.