I don't think there's any harm in having all the Timewalking events up all the time, but I always felt the lack of options in dungeons was a bigger hurdle than the 'cycle' of the expansions and each of them having its own dedicated week. The Mage Tower with new rewards definitely presents a question in that area since it's not the simple "do 5 dungeons for loot" but rather a challenge that may require multiple tries to get.
I don't really mind which way they decide to do it but I'm also curious if the mage tower reward is something you really want, couldn't you just play some more that week when it's active? Even if you are playing another game in that time, it's not like playing a game is exclusive and you can't play others.