Actually, it’s not just for old-world content.
Consider that Soar will still be in the game for future expansions AFTER Dragonflight. If it were overpowered, it might give a real gameplay advantage in progression content in those future expansions.
Of course Blizzard could nerf Soar at the end of Dragonflight… but there would be far more protest if they did so then after everyone had enjoyed it for 2-years, than if they do so now during the Dragonflight Alpha.