On top of that, we've only seen 2 fights in the game that are that big? Or at least people have played 2, that one and the shatterer. We haven't really seen anything from 5-man dungeons for the most part except for a rather poor video of 5 people just doing stuff in an enclosed space, which really doesn't show anything new or interesting.
Yes, being held to the holy trinity can be restrictive to creating content. With the holy trinity you can't make truely dynamic events that are always challenging because you have to take into consideration that a tank might not be there, or a healer. If the holy trinity isn't there, you can make content way more difficult because you don't have to design it around: This is only going to be possible if you have a tank with you, or a healer. Instead you can just make difficult encounters around: Ok, people are here, they can do this. Or, there are 14 thieves here, they can do this.
Enemies don't need threat tables because no one is going to be tanking an enemy, you can do a lot more with a fight just without that restriction alone. Yes, you can be creative when designing a fight with the trinity, I'm not denying that, Blizzard has made plenty of good fights that are different and fun in their own way, but not having the trinity just allows for more freedom, and in return, more room for creativity.