I think a major problem with legion was they assumed you just played every class as an alt so they gave you 15 minutes of quests on your main and then you were like "that's it?". I didn't level alts because of artifact grind and legendary bullshit until the end patch came with instant maxed artifact and legendaries from vendor. That was when I did mage tower on alts. But by the time I was enjoying finishing the mage tower on alts the BFA pre patch came and removed it forever. Only managed to do it on my paladin, warrior, lock, rogue, and feral druid.