Perma-immunity is just a bad game mechanic, period. It needs to be dealt with.
Part of the issue is the timer putting pressure on people to hurry. It's not like it was way back in vanilla, when people would pull enemies away from each other and strategically kite, isolate and pick at their targets.
Nobody does this anymore due to time constraints, all we see are people jumping in head first and hoping for the best. I'm not advocating removing the timer, but that aspect has led to a cautious-less approach to Diablo combat. I feel the true solution is for them to develop numerous, innovative mitigation gears. The new Legendary gem is a good start, but we need way more of that kind of thinking. Perhaps even hybrid gears that give both a heavy defensive bonus, as well as adding something offensive, so that players aren't picking their gear in such a black and white (damage or defense) mentality.
The solution to this problem is GEAR, not game mechanics.