World of Warcraft used boss drops as source of gear upgrades since the beginning. But with Legion they introduced a new concept: the artifact weapon is given by a baseline quest done by everybody and got upgraded gradually by doing various in-game activities.
Many people disliked the change but Blizzard stood by it. In Battle for Azeroth all players wear the upgrading Hearth of Azeroth necklace and 3 pieces of Azerit gear that unlock abilities as the Heart is upgraded – again, via various in-game activities.
I believe it’s not a random change, it is a test run for the post-lockbox World. Lockboxes are banned left and right and Blizzard lawyers probably realized that getting lockboxes for subscription is just as bad as selling them.
How does it connect to boss drops? Because a dead boss is a lockbox that can give out valuable items or
worthless crap rogue items. You gear progress depends more on luck than on actual performance, I’ve just listened to the whining of my GF that she ran all the mythics this week and got only one sidegrade.
Blizzard not offering ways to sell your loot doesn’t help them. There is a living marketplace of accounts that puts value of the drops. That’s gambling and has to go. With the death of lockboxes, all kinds of luck-based progression must go.
Blizzard sees the future and prepares the best way it can: by testing new ways of progression systems. They have time and the Azerite is surely not their final move. By the time they can’t distribute gear based on luck, they’ll have a widely accepted system to replace it.