This is all the more reason why they shouldn't be going away. Just like the legendary questlines in MoP/WoD, some of them are more of a time sink than a test of skill at this point. Taking something like that away at the end of an expansion feels like a cynical way to keep people playing at worst and a participation reward at best.
Given how interesting and unique many of the mage tower appearances are, I think it's really dumb for them to be going away completely. They're perfect rewards to be carried forward and added to other challenging content as an additional incentive next expansion.