When you have something designed to give a flat damage increase like vulnerability, that is what the game is built on: Triggering vulnerability as much as possible and min-maxing your damage through that specific stat.
This was an issue in a game from 2010 called Final Fantasy XIII. Admittedly, Final Fantasy XIII was already a very streamlined game, but the game’s build diversity/variety was restricted even further by the stagger mechanic. Every encounter ended up panning out the same exact way with you pushing a party line-up to stagger an enemy and then swapping to your damage line-up to maximize your damage output. The same thing is occurring with Diablo IV: Trigger vulnerability and then spam dump your core ability/strongest abilities. That isn’t particularly fun and ends up making the game feel more repetitive.
Two things need to happen with vulnerability: It needs to be reworked entirely or it needs to be removed from the game. With the way it currently functions, there will never be a point in Diablo IV where you’re not selecting at least one ability that inflicts vulnerability and subsequently rolling for vulnerability stats on gear.