Why would they leave it open if it literally serves no purpose to anyone and the player base collectively dislikes it?

The Diadem is totally irrelevant. It was supposed to be another way to get great gear for yourself, along with a few other rewards, but players flipped out when it was revealed the PINK gear dropped had an RNG element, and could be BiS. It was unfun FATE grinding for the uncomfortably small chance for BiS.

But, since SE removed that element after player backlash, the Diadem became pointless outside of the exclusive minions and mount. The queues are so comically long I've never been able to enter the place, and now we have those same minioms and mount locked behind an impossible to pass wall that very few can do because no one else has a reason to go.

So, yeah, it's a waste of space and server strain. Can it and put the Hawk and Pegasus drops elsewhere. Put Dev time into new content and learn from the mistakes.