It's so that Thrall can try to establish a time of peace with the Alliance. Naturally it doesnt work. Ofc under Garrosh, that will all change, but isnt BRM geting revamped anyways?

The Horde quest around Moira doesn't need to be changed, as you said, the attempts fail anyway.
But at least these were attempts, visible already in Classic WoW, to settle some of the conflicts between A and H. There wouldn't be a true peace for many years, of course, so battlegrounds would still make sense. But in general, the relations went slightly upwards.
Completely different to nowadays' reaction to player whining that there isn't enough hate er war in the universe dubbed Warcraft after its first game's lurid title - nothing against that, you just don't have to take it that literally.
Warcraft isn't a totally medieval universe with all that xenophobic and/or religion-motivated shit that happened during RL mankind's middle ages. The general philosophy and world view is far further developed than that. Sure, the first game had a simple "us against them, good vs. evil" theme. But already the second one differentiated, explained the Orcs' reason to invade Azeroth and that they were influenced, and finally, Warcraft 3 showed a "good" Horde. But some people still are in the first game's thinking patterns.

I just think an escalation of the current cold war doesn't serve any purpose at all (except caving to above whiners) while there are bigger threats.

Nathanyel's not really being fair to the antagonists of the mediaeval era, but I otherwise agree. Under present circumstances, the ongoing Alliance/Horde hostilities have a hard time NOT looking puerile and counterproductive. Cool as the Wrathgate event was, even it didn't do much to make continued fighting really plausible.

If this is going to continue, Cataclysm will probably need to introduce some casus belli FAR more fundamental & compelling than Garrosh's diaper rash, Varian's bad memories, or Horde lumberjacks.