They explained why already.
They do not want players, who do not do high end content, to go into the next patch, with fully upgraded mythic weapons and trinkets, that realistically they will not upgrade until they can buy the next ones with dinars.
They changed how they want to implement dinars into bad luck protection. Which I think is a good middle ground. All players will have access to heroic items, including the very rare items, that they can fully upgrade.
Currently, if you arent raiding the Mythic Raid, right now, the odds are that you are not going to be raiding it. And the Mythic Raid is something that statistically a very small amount of the players participate in. So, by giving LFR and Normal raiders access to Heroic items, and Heroic Raiders access to some bad luck protection on BIS items, they add a beneficial system for those players.
The same goes for the Mythic raiders. The dinars really only add bad luck protection for them.
With the previous implementation of Dinars, it was because it was the end of an expansion, and the items were spread across three raids. And with a new level cap and a large ilvl increase, the gear would be obsolete early in the next expansion.
Currently, as I said, if they implemented them the same way, you would have a lot players getting gear, that they would not realistically replace from the next patch content, until they get access to dinars again.
Fully upgraded heroic gear is fine for the players not raiding Mythic is my opinion. And I am definitely not going to be raiding mythic myself.
REVISION!
After further consideration, I do think this is unfair for the M+ and PvP community. For all people who play at the highest levels of difficulty, there should be paths to unlocking the mythic track items.
Maybe 3kio and since I am very ignorant of PvP, whatever rank that means. Gladiator? Whatever the equivalent is to approximate difficulty.
Either that or Fully clearing the raid on LFR, Unlocks Normal. Fully Clear on Normal Unlocks Heroic. Then Full Clear on Heroic Unlocks Mythic. And full clear on Mythic awards 3 more coins for another item.
