This exists now … and this is a manufactured problem by blizzard where one doesn’t really exist.
Let’s look at dungeons and the different difficulty level … Let’s face it, normal/heroic and even Mythic 0 (after the first week) doesn’t require any skill, and will never be difficult enough to require CC’ing mobs … obviously, that’s not the point. Players should feel free to round up as many as they can, because again, this content is so easy and becomes so irrelevant to end game content, it doesn’t matter anyway.
But let’s move into keys … As difficulty arises, all trash becomes more dangerous, and the higher key you go, dictates how you handle each trash pack. Where on a mythic +3 for a bunch of Heroic/Mythic raiders is extremely easy and doesn’t need to a slower pace with CC, a +20 or higher will likely require this.
Also, another thing that doesn’t translate well to watching streams is the skill level required to handle AOE’ing a big and dangerous trash pack without CC. Not every group can pull off killing a dangerous trash pack and for them CC is required. But then there are groups that will always be able to because they have the requisite skill. For example, they know exactly which abilities need to be interrupted and which don’t and they execute it well. The tanks know when to start kiting so they don’t die. Healers know which abilities are going to hurt the entire group as to plan a big AOE heal ability. DPS need to know which of the trash pack has a frontal cone that will one-shot them.
There are countless other skill checks needed for difficult enough content, but the point I’m trying to make is, there is not much Blizzard can do to stop skilled groups from rounding up huge packs of mobs. The players have just gotten that good and lesser skilled groups will always need to CC mobs and be more tactical. And with this AOE cap, it’s still not going to change much, because there are plenty of classes that still have uncapped abilities or cap at 8, or soft cap at 5 and DR the rest.
If they truly wanted to prevent this, they would need to hard cap every ability to like 3 mobs at once and that’s it. But that would be terrible and certainly kill the game.
TL;DR
The game right now is very tactical and pulling large trash packs requires skill and coordination for a group running content at the intended item level or more difficult to that. Blizzard, for some reason don’t understand.