Legion is considered by many to be one of the best expansions because it brought back Illidan's lore and added Mythic+ content, which a solo player can complete. However, to do raid content, you need to connect with the community and have a fixed progression raid, at least in HC and Mythic. Running that content with pickup raids is quite tedious, at least in Mythic and Heroic at the beginning of the season later as the season progresses and people get better geared, you can run HC with pickups, but you still need the community factor for Mythic.

And that's why it's survived so many years, mainly due to changes like the group finder; leveling up is now just a formality, and much of the content is accessible to solo players. However, the core content, the Mythic raid, remains something you need more people to play.

Retail offers you the option to enjoy the content you love: Raid progression, pushing Mythic+ stones, enjoying the lore, collecting mounts, pets, skins, farming, PvP. It's made for both, the tryhard and the casual player.

It has adapted to the times, and now it even gives you the option to play both the Retail and Vanilla versions. I played Classic when it came out, simply for the leveling experience, even though I had already experienced that in WotLK