The game is been out for over 15 years(or close to it). They have to change/add things to it otherwise people will just quit. Just like the level squish and then the rework on talents trees from Classic to BC To Warth to the mess we have now and back to talent trees. As the game ages, they have to come up with new to add new abilities but they can't just keep doing that coz you will have so many abilities to use. Instead, they have to reply or borrow powers to make you feel powerful/something change for that expansion and then remove those power so the next expansion they can add something again. You can only add/remove/change a class's abilities so much till you have no choice but to repeat old abilities back in.
Is like tier sets they do away with tier sets coz there isn't much they can add without repeating what was done already in previous expansions. They are basically running out of ideas. i wish they would take a look at other mmo for ideas like FFXIV or GW2. They might not be on the market as long as WOW have but they can rework their class really well each expansion and are able to add new abilities without many issues (given that there is less class and spec in those game but still is something WOW can explore).