I wouldn't be mad about the sun being permanently blocked out, but I'm assuming bethesda wouldn't want to make it permanent, simply because there are players who may change their mind about wanting the sun blocked out, or they wanted to cure their vampirism, which would eliminate the need for the sun to be gone.
In all honesty, so long as there is plenty of notice that it will be permanent, then tough. If they didn't want it done permanently, they should have kept a backup save or not been so dumb. 0.o
EDIT: Howsabout it is permanent, but there is a quest to reverse it if the player so wishes to RP a change of heart and morals?