Imagine this. Say you pay your monthly fee of $15 but it gives you access to not only WoW but other game features such as:
Monthly 5 guaranteed legendary lootboxes in Overwatch [Since there's no subscription required for Overwatch]
Access to all Diablo expanded content.
Access to all HOTS characters.
Access to all Starcraft bonus content.
Retail/Classic Subscription + all previous expansion packs
Hearthstone - Legendary card packs
Warcraft Reforged - New and free monthly added reskins, maps and heroes.