Mana is not an infinite resource, so yes, threat matters
The viability of a group at taking down an encounter is more than one thing, but the tank needs to hold threat, and then stay alive until the boss dies
If the tank doesn’t have enough EHP (uncrittable is baseline, then mitigation, then avoidance to uncrushable if possible, or enough soak to last through a bad strong of crushes + even more mitigation), you’ll wipe, either from spike or going oom from a paper tank
If the tank doesn’t generate enough threat, DPS can’t do enough damage, healers go oom, you wipe
If the DPS spike threat over the tank and die, DPS can’t do enough damage, healers oom, rocks fall, you wipe
They have said the damage is going to get turned up, and the runes on every other tank except warrior have a lot of mitigation built in. Warrior reaches uncrushable very quickly; paladins traditionally just didn’t.
I really doubt threat is going to be an issue for most classes. The biggest issue was mana, and the RF rune solves that one. It does look like they’re going to be stuck add tanking though.