Engineering enchants have been called tinkers since Cataclysm where we also had the cogwheel system.
- - - Updated - - -
They did not know how to use Malfurion. He was supposed to be super powerful but they were afraid to make proper use of him. The wisp wall he erected was definitely amazing (and if the Night Elves and the Alliance had not been hit by the idiot bat in order for the War of Thorns to work, someone would have turned Sylvanas into a pincushion as she was struggling and barely could contain the wall so her forces could pass through). In Cata Azshara herself attacked Darkshore so as to distract Malfurion in order for Ragnaros and Deathwing to attack Hyjal.
Malfurion was simply in a league of his own. The solution was not to depower him but to simply give the Horde people who could be his equals. Thrall could before they decided to retire him, Sylvanas could have naturally been developed to that level, the blood elves should have had at least one magister that was close to Azshara's (and thus Malfurion's) level and the trolls absolutely can produce someone of that skill (Vol'jin is likely heading there).
AS for Merithra it's a shame to have her running to mother. Her quests in BfA were solid and she was on her way to become an aspect just like Kalecgos did, just like Wrathion will try. Instead she is being replaced by an established character who has had a proper arc that finished properly.