Just because the Waystone is intact doesn't mean that Karak Dum didn't fall. There's very large swathes of the Empire that were lost as a result of the skaven wars, their population slaughtered, and have belonged to greenskins and beast men for over a thousand years and their Waystones are still functioning.
Skaven and Beastmen are probably not the best examples here.
Sylvania - Dhar infested wasteland that has remained that way for over a thousand years since the Skaven hit it. The corruption here is comparable to the kurgan steppes and we count that as chaos territory. I think there is a good case most of the Waystones here are gone.
Drakwald - A terrible dense forest unclaimable by the forces of the empire that constantly spews out Beastmen and other assorted monsters. Again this place sounds pretty Dhar corrupted. Waystones probably destroyed.
Forest of Shadows - Took over most of Ostland after Gorthor rampaged over it. A constant haven of chaos cultists and marauders attacking convoys passing through the area. People who venture in jump at shadows and turn to paranoia as the whole forest seems out to get them. Sounds Dhar corrupted to me, Waystones probably destroyed.
Large parts of the Empires Waystone network is gone, that is why the countries further south view us as part of the Chaos Waste.
Right now the deep wastes start around Duum. The chance that taking the Duum waystone moves them far enough south to cover the entire Kurgan steppes is something I think Chaos would take if they had the chance.
We are just not important enough for Chaos to sacrifice their overarching goals for hundreds of years to mess with us personally.
Especially when the next northernmost Waystone if Duum fell might be as far south as Vlag.
I'm not expecting full sunshine and roses when we get there. I am expecting Chaos to have it besieged. For there to be lots of damage to the dwarfhold itself. For them to have taken significant losses. For them to have made uncomfortable choices to help hold the line. For them to be upset that help took so long to come.
Maybe order got pushed back as far as the Waystone room itself.
Maybe only the runes themselves are still holding like on some of the vaults at K8P.
But if Chaos could destroy that Waystone they would have. It is worth more then the demonic backdoor.