This is Warhammer, you don't dick around and play chicken with a prophecy.
Let's say that there is a 50% chance of it the prophecy being bullshit, (Even though there hasnt been anything indication of that in canon and frankly it makes no sense for the nation which constantly gets it's ass hammered by Chaos to kill off half of it's revered magic users just for political reasons when said magic users would go a long way of boosting their forces, if this was a more prosperous nation which isnt constantly invaded by Super Vikings then the it's bullshit idea become a lot more workable. But in a Northern Wasteland besieged by enemies like Kislev, cultural norms would emphasise working together and not killing off half of the incredibly powerful Wizards for no gains, ), that is still 50% chance of it being true and us causing Kislev to get stomped for good, which would be bad for the forces of Order.
And in the off chance that it isn't real, Kislev would still believe that it is real and try to kill our Son constantly, souring our relationship with the Bulwark, and sapping the attention and strength of both our realms away from Chaos and wasted on each other.